1989 Oldsmobile 88 Stalls at low speeds

Engine Mechanical problem
1989 Oldsmobile 88 Automatic 135000 miles

Our '89 Olds 88 has had its Check Engine light come on intermittently for several months, with increasing frequency of late. As of the last month or so, it has been having trouble accelerating from low speeds or uphill. The last couple of weeks, it has been stalling at lights, when we come to a stop. It starts up again fine, however. Two weeks ago, we experienced a "slipping" effect where the car would seem to not engage the next gear when we attempted to accelerate, whether in low or higher gears (highway). We took it to a transmission specialist who said he did not think it was a transmission problem (he also gave topped off the transmission fluid.) We took it a new mechanic--last year we experienced similar problems and our old mechanic couldn't fix the problem, except he installed a new computer chip that fixed the problem for about two weeks, and then it was back. The new mechanic back then gave us a tune up and replaced the alternator, and the car worked OK for a few months. However, now the stalling problem is back, worse than ever, this new mechanic says as long as we can keep it going by restarting it when it stalls, we are better off not looking further into the problem, as this might be costly. The problem is we don't feel it's safe to keep taking the car out while it continues to stall at low speeds, afraid that one day it just won't start again. We've tried to look into this and have spent quite a lot of money on the problem, but nobody seems to know how to fix it for good. The new mechanic does not even seem willing to try. What can we do? What could be causing the car to stall at low speeds? The "slipping" effect when accelerating has diminished since we put in the new fluid, and the car otherwise runs great, but we'd like to get the stalling to go away. Thank you.