BOSTON -- By now there is a pattern to it. The president holds a press conference. He makes a couple of mistakes, a misspeak or two, and a passel of stumbles. His aides stand in the wings looking nervous, occasionally stricken.

When the questions have ended, the members of the media stand around checking their notes and impressions with each other. One is puzzled. Another nonplussed. Metaphors abound. The ship has lost its rudder. The reins have slipped.

Calls go back to the office. Editorial conferences are held. Is it time for another piece, segment, show on this? Do we ask again the questions about Reagan the non-manager? Is it time for a Gipper-grasp update? Anybody got a new way to handle the delicate issue of age and slippage?

Somebody in the conference groans, ''Old news, we've done that piece.'' Somebody else insists, ''But you're talking about the president of the United States.'' The segment, piece, show gets done somewhere. It includes some stumble clips, some reactions and careful commentary.

Next come the pollsters, who send their questions out over the phone lines and come back with the reactions of the public. The bottom line is: they think the president is doing a good job.

This scenario -- run after the first Mondale debate, rerun during the winter of Iran revelations -- was recycled yet again in the past week. As the economic summit broke up, the president rambled through his Venice press conference. He could not remember the name of the United Nations Security Council. He didn't know that the Germans had already decided to try hijacker Mohammed Ali Hamadei. He forgot that his policy was to stabilize the dollar.

Details, details. The shovel brigade came out in full force. There was a media event at the Berlin Wall. There was a speech from the Oval Office. And all is quiet in public opinion.

By now, I have developed a theory about this silent majority. I don't think Americans are unaware that the presidential grasp of information, the presidential performance, has diminished over the past seven years. I think something else is going on in the public mind.

Call it the Favorite Grandfather Theory, if you will. If the analysts are right, many Americans have regarded Reagan as a father figure or a grandfather figure. The young in particular voted for him in droves. The majority have been -- there is no other word for it -- fond of the man.

But as he slides through his late seventies, under the enormous pressure of office, their respectfulness is also sliding . . . into protectiveness.

A friend and Reagan supporter tells me that watching the president perform without a script makes him anxious. It's not exactly like watching his child at a piano recital, waiting for the fingers to slip. It's more like watching an elder, a mentor, yes, a favorite grandfather, losing his powers in public. My friend closes his eyes and turns off his TV at the scary parts.

I find in myself a similar and unfamiliar reaction. When my colleagues press the president, I subconsciously hold my breath: Will it happen now, is this going to be embarrassing? I don't want to see the man lose his dignity.

This is tough stuff to talk about. But I think these squeamish feelings are widespread. Lesser fans of Ronald Reagan than I find themselves perversely hoping that he'll just get through the next press conference, the next summit meeting -- the rest of the term -- and retire happily to his ranch. This hope mutes our criticism. Mutes our demands. Mutes us.

The Favorite Grandfather Factor is not something that people share with outsiders, certainly not those outside the family. Certainly not pollsters. Families have the instinct to protect their patriarchs, save their pride, and Ronald Reagan has elicited family feelings from millions in this country.

So it goes on. By some unwritten agreement, the president is protected and the public protected from acknowledging his decline. Every once in a while there comes a press conference, another handful of mistakes. Questions about his performance resurface. And then they pass and, for a little while longer, we are spared the discomfort of confronting what we already know: the president of the United States is past his prime.