my brake light comes on once in awhile while driving just because my parking brake system is a bit f'ed up (one bellcrank is frozen and the cables seem a bit stretched), so it doesn't have enough tension to hold the brake lever all the way in.

so, if i shift too hard or hit a bump too hard, the brake lever sometimes pulls itself out a bit and causes the light to come on. i just push it back in (or hit another bump), and the light goes off.

not exactly the same symptoms you have, but easy to check.

stan

1994 4x4 22RE W56 truck