2004 Chevy Cavalier misfiring

I have an issue with a 2004 Cavalier, when at an idle in feels as if it is going to stall, the RPM go from around 700 and drop sharply to 500, at this time the dash lights flicker slightly also. When driving the car there is no issue it rides just fine once the gas pedal is depressed. Also when the car is at an idle there is sound I can only describe as one fo those garage doors that open and close by pulling a rotating chain, only really fast. Now for the things that have been done to try and remedy the problem are as follows: The spark plugs have been changed, there has been fuel injector cleaner run through the car, the throttle body has been cleaned out, the IAC motor was visually inspected and cleaned and it appears to be fine, the coil pack doesnt seem to have any issues and autozone has pulled a code out of the car p0300 which doesnt help much. I have been told that possibly the timing has been thrown off and that all that junk in there needs to be changed. Which is fine, but it is expensive as is all the rest of the possible fixes for this car. I guess my question is how likely is this issue to be, the ERG valve, fuel pump, timing issue, a blown head, or anything else I havent found in any research I have done. Thank you in advance for any help you can offer.

Check the TPS