1990 Cadillac Deville Timing or Fuel Delivery issue?

I bought a '90 Cadillac Coupe Deville that has 126,000 miles on the original engine. It had a gas leak come to find out. The fuel filter was replaced and we fixed the line where the fuel filter connects. Then there was another gas leak where the sending unit is. I bought one online and replaced it. The fuel injectors were all replaced and a tune up was given. The car sometimes hesitates and it feels that there's pressure behind the pedal when driving at highway speed. This started after the tune up was done and that was the last thing that was worked on the car.

The car idle isn't right either. I had the catalytic converter tested and it needs to be replaced and I am doing it this week. The car when at a stop or if it is idling, feels like the gas is pulsating under the car from the tank to the engine and like I said, it idles rough (but not as much as before when I bought it) and it will eventually die within a minute or two or sooner. It'll start right back up, but if I don't rev it at a light with one foot on brake or one on the gas, it'll cut out. The check engine or no lights appear on the panel (except for the air conditioning one when you turn on the defrost or vent.) The colder weather, it seems to run a little better or is more responsive and less pressure behind the gas pedal, however the pulsating underneath the car is still there. If I turn on the defrost or fan, and if I am not at highway speed, the car dies too or if I go from reverse to drive. I do know the air conditioning isn't working, because the belt isn't hooked up right now to the compressor. I just wanted some advice or suggestions. Thanks.