My 1991 325i 2.5 M30...

It runs great. Nice acceleration...no problems really.

The engines surges when I am idling at at stoplight. I can slightly press the gas pedal and raise the rpm's to level out the engine.

Is this because I run 93 octane gas? Or is it a vacuum problem?

Vacuum leak. Its not an M3 so I'm not sure on the location of the lines for your motor, but check the vacuum line from the intake manifold, that is likely one of or the only cause. Also check the one right at the IAC mechanism.

Likely also that your icv is dirty.

And it's not an M30 btw. It's an M20b25.

I saw you have another thread in here.....you may want to move to the regular E30 forum not the M3 section, you'll get more response. And it's the correct section