Engine not going over 1000 RPM and falls back down

@tillhimself

does the engine ever go over the 1500RPM even if the transmission is in park or neutral? This does not sound like a transmission issue but either a fuel or sensor issues. I’d check things like bad crank shaft, cam shaft position sensor, MAF as well as a throttle position sensor. It is of course also possible that your car is in limp mode. You need to give us more information as to how long this has been going on and if there was anything that preceeded this. You also should consider having the OBD scanned to see which codes you are getting.

Disconnect the battery and wait for 15-30 min then reconnect. (make sure you have your radio code if your radio requires one)See if that changes anything.