90 Celica idling high...what's the cause?

I've got a 1990 Celica GT with the 5SFE 2.2L 5-speed engine. Ever since I've had the car, it idles all the time from the lowest of 1500rpm to the highest of 2000rpm. The lowest I've ever seen it idle is 1000rpm and that was for 3 seconds. It does that sometimes. It's like the rpm's just dip down to 1000 for a quick couple seconds then they rise right back up to 1500-2000. I've done some basic troubleshooting such as spraying carb/brake cleaner around possible leaking points like the intake manifold, injectors, vacuum hoses and lines, and valve cover. But spraying those points does not seem to make the engine's rpms change. I'm pretty puzzled as to what could be causing this issue. And since this is an OBD0 car, I don't have a scanner that will work with it. Also, this sometimes works and sometimes doesn't, but I can tap the gas pedal real quick and the idles come down to around 1500-1600. Doesn't always work though. I've checked the idle screw on the throttle body as well and it's screwed all the way in. If I back it out at all, it makes the engine idle even higher. Any thoughts or suggestions from anyone?