2003 Pontiac Grand Am RPM's dropping while driving

The engine light has not come on, the fuel filter has been changed, as for the fuel pump not sure how to check that I do know that is ingageing when starting you can hear it kick on, also there is no hesitation in starting so was hoping to rule that out, I also had the tranny flushed and new filter put in thinking is was slippage in the tran but that is now ruled out to,
I did get a new catalatic convertor put on because someone actually stole it off my car and they put a universal on my car it was cheaper since I had to pay for it, could that cause the fluxuation in the RPM's
thank you for your help
Kristal