I have a 85 honda prelude. it has only 40,000 miles. it surges at steady speeds. it does not surge when giving acelleration.

SOURCE: Air conditioner fan motor speed surges

needs a new blower motor controller

SOURCE: surges or bucks at steady speed, 40 to 50 mph

change fuel filter have injection system cleaned

SOURCE: 1998 Pontiac Grand Am GT 3.1 V-6 Engine RPM Surge

might be ur throttle position sensor

SOURCE: 1991 Caprice Classic 305 engine surge

Sounds like the throttle position sensor to me. I have a 1991 caprice wagon and I have to replace this part every 6 months or so. It's a cheap easy to replace part, around $25, but I haven't been able to find one that will last like the OEM. It's just a variable resistor that tells the computer where you have the gas pedel. When it fails it tells the computer you're giving it gas when you're not.

SOURCE: 1989 honda prelude. engine surges. from normal to 2100 rpms

There is a warm up valve on the throttle body(should have 2 small coolant hoses). remove the valve and have someone start the car while you block the port to the valve on the intake. If that clears up the problem you can cut a piece of cardboard to mount in between the valve and intake.should straighten it out.