Surge, surge, surgin' when warm

HELP!!
Have a '93 2-door Geo Metro Automatic 3 cylinder. We found out after purchase that it has a '90 engine. The car was manufactured in Canada, if that makes any difference. (Don't know where the engine was manufactured.) I only had this car 2 or 3 weeks before this problem started.
Engine begins surging while holding accelerator steady. Doesn't seem to do it until it warms up (A couple of minutes). This is a steady pattern.. up,down,up,down.. about 1 second or a little longer for each cycle.

When you look down the fuel injection port when it's doing it, it doesn't spray during the pulses.

Also, the thermostat for the engine does not kick on the fan. This didn't change after we replaced the thermostat with a new one.

Since buying this, we have replaced the following with new parts:
replaced the thermostat for the antifreeze.
spark plugs & wires
distributer cap and rotor
fuel filter
Also, we replaced antifreeze temperature sensor with a used one. (noticed
at the junk yard that there was another sensor right behind that one, which
we did not replace)

A lot of repair shops don't want anything to do with this car, because of the body/engine mismatch. No one seems to be able to test the computer except for Chevy, which charges $94 just to diagnose the problem!!

The computer (ECM??) looks like a BITCH to get to, under the dash and behind fuse box and more, if we were going to try to switch it out for one from a junk yard.

Can anyone help, PLEASE! Anyone had this problem themselves?

Thanks so much..

Mike & Martha

Welcome to AF. Check out the forum search function and you will often find clues to what to check. I ran a quick one by inputting "surge" and found the following posts that might help.
http://www.automotiveforums.com/vbulletin/showthread.php?t=465161&highlight=surge
http://www.automotiveforums.com/vbulletin/showthread.php?t=373660&highlight=surge
http://www.automotiveforums.com/vbulletin/showthread.php?t=345850&highlight=surge
http://www.automotiveforums.com/vbulletin/showthread.php?t=282899&highlight=surge
Good luck on your quest.

Try replacing the throttle body gasket.
That's what made my car surge at idle.
Test it by spraying carb cleaner at the gasket joint while running. If spraying changes the speed, you have a vacuum leak there.

Sounds like a faulty ISC Solenoid.