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

 

you must narrow down the problem ..it could be a countless number of issues causing this ..here are the main cause I have heard:
first check for vaccum leaks as 170k can cause many of your hoses to be brittle and lose vaccum. ( lightly spray some carb cleaner around all the hoses on a warmed up and running engine and see if you get any idle fluctuation.. be sure to avoid the exhaust manifold!) After you confirm no leaks next check your TPS Setting ( throttle position sensor) ..there are many posts on setting the TPS so I will not go into it. Next I would check the Bypass air control valve ( BAC valve ) pull the hose off the bac valve that connects to the intake after the air filter box while car is not running ..cap the BAC valve so no air can enter with a internal plug and reconnect the hose. Start the car an see if the Idle still fluctuates ..if not your BAc valve will need to be cleaned or replaced. let me know the outcome ...good luck!

 

Wow sounds just like the GTU I bought except mine ran like poop as well.

Why isnt this in the 2nd gen section?

Anyway I had a 91 years ago that sounds similar and it was a combination of things, vacuum leaks mostly caused by intake gaskets needing to be replaces, fuel injectors needed to be cleaned and needed new o-rings (forgot what their called, the bottom ones) AND THEN FINALLY AFTER ALL THAT some adjustment to the screw on top of the throttle body. You can also check your TPS but you have 2 of them so I dont remember the exact proceedure.