Hi. I have a 2001 WRX. I change standard pistons with forged ones. After this I start the engine, let the car warm up, and then turn it off, wait for it to be cold, and refill coolant. Now I start engine, let it warm up, and then rev it to 4000rpm, but about 2 minutes after the rpms fall to idle and then keep balancing between 500 and 1500 rpms until I lift my foot from accelerator and then if I accelerate it goes again to 4000rpm for about more 2 minutes and then it fall again to idle rpm and so on so on... What could be wrong.