Car is sputtering and spitting out smoke

My auto is a 1994 Cdn Sunbird LE with 143,000km on it. The other day I put $20.00 worth of gas in it, not my regular gas station. They I took it in to have the oil changed. After that I was driving along and the front exhaust pipe fell off. I took it to a muffler shoppe and they replaced the pipe. My car is now sputting and spewing and almost stalling while it idles. I took it back to the muffler shoppe and he was quite insulted that I thought it might be his work. I am checking all possibilities at this point. Could it be the PCV valve, the timing chain or is it the pipe. It feels like it is not getting any gas. I put fuel injector liquid in the tank and some gasline antifreeze but this did not help at all. What could be wrong with my car. It always purrs like a kitten.
Thank you
Jodyree
Your form will not accept my donation of $10.00 for some reason. If you can tell me how to do it I will donate, but I have now tried four times.