It sounds like you have the revlimiter from an Eta series. They have a soft valve train and a limited revlimit to keep the valve float down in an attempt not to break anything. It might just be your ECM, look up the last 3 numbers on it. If thats the case i think you can swap it out for an I series ECM as long as you change the sensors as well. There was a very detailed write up on E30 tech or revlimited, i'll have to go find it....