Engine will sometimes not rev

[NoDOHC]

Best bet is try unplugging the OMP and see if you get the same behavior consistantly (don't let it run too long with the pump unplugged) You could try cleaning up all the connection points between the ECU and the OMP.

I think that if I remember correctly, there is a motor in there to adjust the position and a potentiometer to indicate position back to the ECU (closed loop control) If the resistance does not react as expected, you can get the code. You could have corrosion on the wipe contacts of the potentiometer too (a 'dead spot').

I don't know how to tell you to test that.

You could always try a different OMP, I have one, but it hasn't been on a running engine in years.