1990 Century ...computer problem

The computer (I guess) on our 1990 Century wagon has been acting funny lately. For no apparent reason, the "Service Engine Soon" warning light comes on and the engine stalls after 10-15 seconds.

I can count to 10, fire her up again and she's good for another few (hundred) miles.

I had a mechanic run all the necessary tests but he can't find a thing wrong ! Any ideas ?