Buddy Brown and his mount, Jet Lag, soared over a dozen jumps today to win the intermediate-jumper class and continue their domination of the jumper competition at the Upperville Colt and Horse Show.

Brown, 24, of Palmetto, Fla., who won the A section of the preliminary-jumper class on Rufus, was one of eight riders from an original field of 19 to qualify for a timed jumpoff.

Brown's winning time was 36.443 seconds abroad the 16.2-hands 7-year-old chestnut gelding owned by Patrick Butler of St. Paul, Minn.

"He's very easy to ride," said Brown, a member of the U.S. Equestrian Team since 1974 and a member of the 1979 Pan American Games gold-medal team. "He's been in here and at Devon (Pa.) last week."

Brown now has won all three preliminary-jumper classes.He won the first two on Harry Gill's Time To Jig, a horse he refers to as "a nice guy and a good friend."

Charlie Weaver, 27, of Keswick, Va., continued his domination of hunter competition, winning two championships.

Weaver rode Ruxton, owned by August . Busch Jr. of St. Louis, to the championship in the regular working hunter with 21 points. They also were winners last year. In the small working hunter division, Weaver was also a repeat winner, guiding Super Flash to the championship with 20 points. Super Flash is owned by Mrs. Sylvester Johnson of Zionsville, Ill.

Judy Lee of Keswick repeated her Devon victory of last week in the amateur-owner division, taking champion and reserve with Like A Charm and Winning Hand.

Alan Smith of Middleburg won both open jumper classes today, on Rendezvous, owned by Caroline Clark o f Iron Station, N.C., and Ingleside Stables, and Fair Cast, owned by Diane and Joe Fiore of Middleburg.