Help with 1994 Nissan Altima, 4.2L, wants to stall

Car runs fine, and all the sudden the RPMs drop, but it doesn't stall.  This is a random occurrence.  Sometimes when I put pressure on the gas, I notice it happen, dropping RPMs, but some days it doesn't.  I have no check engine light or codes coming up.  I have changed the spark plugs already.  I have changed the catalytic converter, knowing that's one of the components that will not trigger a code.  I still have the problem. I have added some C foam to the gas to clean the injectors, and I'm going to change the spark plug wires and the gas filter next.  Hopefully this does it.  What else do you suggest?  Checking the gas pressure in the pump? Am I on the right track?  Suggestions?  Thanks.