After an NFL season full of such boring variables as third-down conversion percentages and time of possession, it's a relief to have a playoff game where the key ingredients are wolfsbane, dead frogs and armadillo toenails.

Of all the whammies now operative in the NFL, the rex the Minnesota Vikings have over the Los Angeles Rams may be the most chilling and total.

If the bedraggled Vikings beat the Rams again here Sunday in their 5 p.m. first-round playoff game (WDVM-TV-9), the Minnesota team plans should be searched for warlocks.

No pundit can dream up a way that the mediocre (8-7-1) Minnesotans with their rebuilt defense and rushless offense should handle the Rams (12-4), who have football's No. 1-ranked defense.

But that's always the way it seems before the Vikings start stirring their caldron of blocked kicks, lucky bounces, dubious calls and icy Minnesota poise.

Four times since 1969, three times in the last four years, the Vikings have ended the Ram season with a playoff beating.

If it weren't for the tradition, this game might lack tension. With it, however, this game has no choice but to be distinctive-either a Ram semivindication or the most amazing of all the Viking upsets.

Last year Minnesota came here without quarterback Fran Tarkenton and with the memory of a 35-3 regular season licking by the Rams. It rained. The Rams choked on the mud and their own anciety to win. And Minnesota escaped, 14-7. L.A. Coach Chuck Knox lost his job.

This year's Viking team is only an eight-point underdog, which proves that oddsmakers are both superstitious and respectful toward any team with a healthy Tarkenton.

This game offers a delicious buffet of contrasts-walk down the line and take your pick.

The Rams, the team that never gets to the SuperBowl, against the Vikings, the team that always wishes it hadn't.

Rams first-year Coach Ray Malavasi who has been annoying the Vikings all week by denigrating Tarkenton and pointing out how difficult it will be for his Rams to lose, against silent and savvy Vike Coach Bud Grant who was so nervous about L.A. that he gave his team a week off after their last regular-season game.

The still-cautious-after-all-these-years Ram offense that drives its fans crazy, against the Tarkenton air circus that has launched 592 passes this year.

The Rock of Gibraltar Ram defense of Jack (Hacksaw) Reynolds, Jack Youngblood, Jim Youngblood and Fred Dryer against the Viking defense that no longer has the reputation of the voracious Purple People Eaters.

The contrast leading up to this blood feud has been starting. The Vikings, ensconced in Tucson, Ariz., for practice, say only that their many injuries are healed and that they plan to play the way they did this season in beating such strong teams as Dallas and Denver.

The Rams, au contraire, have been biting and scratching everything in sight, including each other.

Desposed L.A. linebacker Isiah Robertson, and unpopular dissident all year, got into a scuffle with lineman Ron Saul. Saul choked him until he passed out.The next day in practice, quarterback Pat Haden, aware that Robertson's number is 58 and Saul's 61, called the day's first play: "Sixty-one choke 58, on two."

Very amusing. But when an L.A. reporter asked Robertson for a rundown on the sequence of holds that sent him to the land of nod, Robertson took a punch at the writer, and, naturally, missed.

Ram owner Carroll Rosenbloom, sided with Robertson and helped eject the reporter from Ram practice-or an least remove him to higher ground.

Meanwhile, Haden was thinking about the game. "My mind is on the emotional state of my teammates and the strategic weaknesses of the Vikings.

'I can sense that (lineman) Doug Francis can't wait for me to run plays over him. He's at the crazy-psyche level already. When I'm dropping to sleep, I visualize the Viking defense. I know they've got a rookie cornerback on the right side with (Nate) Wright out. I know the other corner, Bobby Bryant, is getting older, but he's a big-play man. Can't let him burn you. Their linebacker Jeff Siemon is small but he's so smart that you can't go in the well there too often."

In some sense, the Rams have already lost a good deal before this game starts. As did the Dodgers across town during the last World Sories, they have shown a world-class talent for retroactive alibis.

"We've always gotten the short end of the officiating or the luck in the past," says Malavasi, adding rather gratuitously, "I don't think Tarkenton is as effective as he was two or three years ago . . .

"It none of my business what Bob does . . . but I believe the layoff he gave them was too long."

Tarkenton, who broke the NFL season record for completions, using an offense that was devoid of a running attack, answers simply. "This is the best season I've ever had . . . We've done more with less . . . As long as you're playing quarterback, you'll be criticized. Just watch, I'll become a whole lot greater quarterback after I've retired a few years and nobody has to watch me play anymore."

Nevertheless, at least one Ram coach thinks this game will come down to those old ingredients-Tarkenton and wolfsbane.

"The Rams have been the most unlucky club I've ever seen," said Paul Lanham, first-year special team coach, for years a Redskin assistant. "The Vikes beat them with experience. Tennesse game savvy . . . Missesota was just like Miami, Oakland and the Redskins. You knew those teams would find a way to steal the game, even if they didn't deserve it. Those old Viking teams were endearing to watch . . . but most of them are gone now.

"They always beat the Rams with Tarkenton drawing some sandlot plan in the dirt, or their special teams killing them with a big play. Their special teams are still magicians, but now ours are kind of good, too."