SAN BERNARDINO, Calif. (AP) _ Shandra Percy got stuck in her garage Friday, and like hundreds of other homeowners hereabouts she blames her problem - an automatic garage door that refuses to budge - on President Reagan.

They believe the doors' radio controls are jammed by signals from the president's E-4B flying command post, the ''doomsday plane'' that parks at nearby March Air Force Base when Reagan vacations at his California ranch.

''It happened this morning and I said, 'He must be in California,' '' said Ms. Percy, who during past presidential visits has shut off the automatic opener and worked the door by hand.

The originator of the plane theory, gargage-door repairman Larry Murdock, said that since Reagan and his wife, Nancy, arrived Wednesday, he's answered more than 35 calls from people whose doors refuse to respond to remote-control transmitters. He charges $10 to $30 to change the openers' radio frequencies.

Murdock's Genie Garage Doors has received far fewer trouble calls than it did during the first family's visit in April, and Murdock says he knows why:

''Would you believe I changed 485 frequencies last time?''

The administration and the Air Force have refused to discuss the possibility that the plane's signals are to blame.

Murdock, 57, who holds a doctorate in physics, said San Bernardino, 55 miles east of Los Angeles, would be in the path of FM signals beamed from the air force base to Reagan's ranch in the mountains 160 miles to the northeast.

Murdock, who voted for Reagan, says he defends the president against customers' complaints:

''I tell these Democrats, 'Don't blame him.' It could have been a Democrat up there on that hill causing the problem.''