90 100q stalls out

[syljay at optonline.net]

My 1990 100q, manual, non turbo stalled out at the mall.Pre-existing condition:Car has an exhaust leak. I'm pretty sure its at the downpipe/cat flange. The leak noise goes away after the car warms up. Last time I was under the car, about 6 months ago, I left the flange bolts/nuts a bit on the loose side cause the downpipe flange was rusty, thin, and bending. I suspect with a bit of bouncing of the exhaust system, the flange worked its way a little loose. When the exhaust pipes are cold and contracted, an opening appears. After a warm-up, the pipes heat up, expand and seal off the gap.I've had this exhaust problem for about a month now . .. with no stalling issues.Sequence of events:Went to the mall on Christmas Eve in the 90 100q.Drove to the first store with no problems.Came back after about 45 minutes and drove car to next store about 1 mile away. No problems.Came out of the store about 30 minutes later - started the car - and proceeded to drive out of parking lot.I felt something wrong, the engine would buck/miss if given too much gas. I looked for an empty parking spot and creeped towards it. Engine was missing badly and finally stalled out. Engine would not start, cranking was fine. We got some good samaritans to help push the car into the parking space.I called wife who  picked me up in the 88 5kq.Two hours later, I went back to get some things forgotten in the 90 100Q.I swapped out the Fuel pump relay with the one from my 88 5kq. Car started right up. I put back the original relay, car started right up.I went home, got a spare fuel pump relay, and drove back to the car with wife in tow.Put in the spare relay, started the car - no problem.Let the car warm up, and drove around the parking lot a bit.I felt a hesitation a couple of times when starting off and giving gas. I let it warm up some more and tried again. I was easy on the gas and the car seemed to work ok.I decided to try getting the car home.All worked ok, until I got to a stop light just before the entrance to RT 80. As I  was starting off, the car bucked and I thought it stalled out. Being in traffic, I didn't watch the RPM gauge and tried starting the engine . . got a grinding noise . .the engine was not stalled.I drove off, being gentle on the gas. By now I was suspecting that giving too much gas too quickly resulted in the bucking/cutting out. Maybe the engine torque and engine rotation was moving something.I drove all the way home (about 10 miles) on the highway with no problems  - - being gentle with the gas.And now, for the $64,000 question - Whats the problem?SJ in NJ