I have a 90 GTU I just bought and it has 170,000 miles and dont I know any history about it. The Idle constantly jumps from 1,000 to 1,500. the idle seems to be always going up and down. The idle adjustment on top of throttle body has also been messed with. I'm pretty sure the aux ports are clogged because doesnt have the power it should but not sure how difficuilt to clean. Also it has a glass see through sunroof, I was told that it didnt come with it but there is a mazda sticker inside the door that says dealer modification

 

There are a lot of things that can screw up your idle. If your aux ports are stuck closed, then it shouldn't bother it. If they are stuck open, then it's getting too much air at low RPM's. The actuator pulls the valve open. If the shaft is extended (approx. 1 1/2" of shaft showing) then the ports should be closed, providing the linkage is intact. It's difficult to say without removing the intake manifolds and looking at it. With that many miles, you could have internal problems. I would make sure all the vacuum lines are in good shape, and in the right spots. The manifold bolts are tight. Check the TPS per the FSM, and then adjust the idle. Then you can check your BAC, and grounding. If you do a search for "idle" you'll read all about it. Have fun.