1992 Buick Regal Custom coil packs

1992 Buick Regal Custom, mileage 145,000. We recently replaced our engine and within a short time, it acted up on my wife in stop and go traffic. The people who replaced the engine then replaced the coil packs. This seemed to work for a while, but it started to backfire and lose power when my wife was in stop and go traffic last week. I was kind of not sure that it was doing this because I wasn't there. I drove it to and from work for about the next week and a half and last night it did it to me. It was backfiring through the carburetor I think and I didn't see any smoke. When I tried to give it more gas it acted like it had no power at all and soon died. I let it sit for about ten minutes and then I drove it the rest of the way home (about a block and a half) and it started to do it again just as I got in the driveway. We don't know what to think and we're not sure if the guys who replaced the engine know either. Please help! We have only put about 4,000 miles on it since the engine was replaced. This has put us $4,000 in debt.