Engine stumbling

When I press the gas, both in an out of gear (eliminating the transmission as the issue) the car stumbles around 3000 rpm. It does not want to go any higher, but if kept under 3000 it runs perfectly fine. The fuel filter and fuel pump are new, as are the spark plugs and wires.
The engine is rebuilt and the heads are new, along with the water pump and belt.
I am wondering what the issues could be? I am thinking it has to be either and air issue, or fuel injectors or a vacuum seal, and if nothing else, some sort of ECU issue.
I am hoping to find out what is most likely to be the problem (I know it is impossible to diagnose over the internet).
mainly want to know if there could be any larger issues here, like engine internals or something expensive to fix?

Thank you!