i have a 1991 accord lx and when in park or neutral the idle goes up and down but in drive or reverse it doesnt. anyone had this problem?

 

It could be you're Idle Air Control Valve on the backside of the intake manifold being gunked up with carbon and not opening. This is what allows air into the cylinders when the throttle plate is closed (foot off the pedal)

 

Or a PVC Valve... which is really cheap and easy to fix.

 

Fixed it for ya ;-). Try PCV first, they're like $4 max. I personally would get a NAPA one, all others I've gotten tend to break apart.

 

On my old Integra, i had the same problem, I cleaned the IACV with Carb cleaner, and it worked fine after that, tweeked the distributor a bit to get the perfect idle.