Engine won't rev past 4000 rpms, please help

I have a 1998 Grand Prix with a 3.1 and high miles. The service engine soon light came on about a week ago. Now the engine won't rev past 4000 rpms in neutral, and is shifting weird, and doesn't have very good acceleration at all. I brought it in for a computer scan, which came up with:
PO122, TPS circuit low input
PO300, random/multiple cylinder misfire
P1122, TPS intermittent ckt

I replaced the throttle position sensor, drove it for a little bit, the light came back on, then scanned it again, with a result of PO300 again. I did a general tune up, replacing plugs, wires, fuel filter, air filter and replacing the engine control module and ignition coils, with no difference in the revving problem.

I will bring it in again for another scan tomorow, but I'm expecting to get nothing with the new scan. Does anyone have any ideas of which other sensors, switches or module type things this could be? Something that wouldn't set off the service engine soon light? Maybe the Crank position sensor? MAF sensor? Could it be that the catalytic converter is plugged? EGR valve? Thanks in advance for any help recieved. Scott

Im pretty sure there is a rev limiter that wont let u rev past 4k. Really you shouldnt be trying to rev the motor that high w/ no load. Good way to spin a bearing

I would suspect a bad injector or a bad cat. You didn't say how many miles are on the vehicle but it certainly should rev beyone 4k in neutral even though it may not be a good idea to do so. If you have already taken care of the ignition side of the system then the problem has to be in the fuel system. Therefore, check the injectors to see if they are working properly. If the computer points you to a certain cylinder, that is where I would start. Also, the egr valve on the 3.1 is not hard to clean so you may want to do it anyway. Don't forget to clean the passage as well.