I'm a new owner of a 88 rx-7 and recently the RPMs have been running real low almost like its about to stall and then when I accelerate I feel as if I'm not getting the same power that I used to and at a constant speed the car kinda jerks as if I had it in too high of a gear for that speed. I just replaced the muffler system and I have a new clutch. Does anyone have any idea of what it could be?

I would check the throttle position sensor. Not exactly sure how yours works but most are fed with 5 volts. And the signal wire going back to computer will read from .5 to 1 volt at idle to 4.5 to 5 volts at full throttle. Should be a smooth increase and decrease in voltage as the throttle is moved in either direction. Hope this helps.

Start off with checking for error codes (http://www.fc3spro.com/TECH/HOWTO/ERRORCODES/ZENKI/zerror.html is a good guide, takes some bits but worth the work). If there are no error codes present then check the air filter for clogging and then check for vacuum leaks and for correct ignition timing. If that's not your solution you will need to get the FSM out and check the following things: MAF, auxiliary air port valve, injectors, TPS, MAP sensor, fuel pressure, coolant temperature sensor. If none of those pan out, you may want to check the compression. Personally I would suspect the intake air system as the primary suspect, then the ignition and finally the fuel system. By the way, what sort of exhaust system did you install? It might be the cause of your problems, but normally that is the last item I check because it hardly ever causes trouble unless it's banged up in such a way that it badly restricts exhaust flow.

If you changed the exhaust out and didn't change your O2 sensor check that. When I switched to my header in the process of removing the O2 sensor it shorted out on me. It just takes a tap on metal apparently because my brand new one shorted out putting it in as well. Made my 88 run like poop until I got a good one in. I don't have any emissions crap left on mine so I don't run in to the other issues they mention.