1990 Chevy Lumina Hart to start cold or hot

Just had a new pipe and muffler installed. I have been having trouble starting the car since that day. Perhaps just a coincidence, but it's puzzleing. Car runs awesome. Lots of power. Idles fairly smooth. Within the last 2 years, new fuel pump, crankshaft sensor, plugs, wires, fuel filter, airfilter, AIC motor, PCV valve and ECC. Tough to start weather it's cold or hot, I have to make several attempts before it starts, then it starts but not smoothly for a second or two, then it's fine. I thought it could be the fuel pressure regulator. I disconnected the vacuum hose, no fuel odor or leaks. I have no way of doing a pressure test myself. My old exhaust system had many leaks, and the car ran fantastic. With the new exhaust, the car runs good, but a little slower when accelerating fast, but there are no issues with power or acceleration. The car never stalls. Doesn't burn oil. However the exhaust odor is a little strong. Someone mentioned to me it smells like a rich mixture. Bad O2 sensor? Bad fuel regulator or leaking injectors? Could the new exhaust have anything to do with the problem? I have noticed when it does start, if I shut it off for a minuite or two, it will restart just fine. I love this car, and will never get rid of it. I want to keep it running forever.