I've fixed the cause of this in my car but am posting the steps I went through to maybe help someone else frustrated by this problem.
I first noticed there was a problem when the cruise control wouldn't stay set. It would suddenly cut off. So, I started by pulling the codes and found the infamous 17990...throttle body adaptation limit reached. I first tried removing and cleaning the throttle body which wasn't very dirty at all really.
This had no effect and the problem continued to get worse. Now I also had a bouncing idle. The car would sometimes idle normally, but would suddenly begin to bounce between 600-1200 RPM's and had a dead spot in the throttle just off idle. So, I continued down the list of things to check...
Check Fuel Pressure
Check Intake System for Leaks (False Air)
Check Fuel Injector(s)
Check Evaporative Emission (EVAP) Canister Purge Regulator Valve (N80)
Check Exhaust System for Leaks
Check Secondary Air Injection (AIR) for Leaks
Check Vacuum Lines for Leaks
Check Throttle Body
The only thing I couldn't really check was the fuel pressure since I don't have the equipment for that, but everything else checked out fine and running a throttle body alignment had no effect either.
What ended up being the problem was the throttle body itself, or more specifically the throttle position sensor built into the throttle body. Once I replaced the throttle body and ran another adaptation all is back to normal. And for those who say you don't need to run the adaptation all I can say is that I drove mine for a couple weeks with the new throttle body and did the battery disconnect, turn the key on and leave it for five minutes trick and the car never did learn the new throttle body on it's own. TBA is mandatory if the throttle body is changed. Maybe that shortcut will work if you are simply removing the old throttle body for cleaning, but if it's changed for a new one I'm thinking the ECU must be aligned to the new TB. If you don't have the full version of Vag-Com you'll need to find someone who has it to run the adaptation. There are some awesome folks on here who are willing to help out however they can. I found someone near where I live and I can't begin to say how much I appreciate his help getting my car back to her old self.
Hopefully this write up will save someone some of the frustration I went through searching for answers and the cause of this issue. My car is now running like new again and ready for another 150k.
1998 GTI 2.0
1993 Jeep ZJ 4.0
Need any VCDS (VAG-COM) diagnostics or coding in the North NJ area? PM me.