I have a 98 T5 and it has about 160,xxx on it, and its previous owner always put 87 octane in and never really kept up on much maintenence but... It starts up perfect everytime but when its idleing the rpms range a couple hundred when its just siting there. You can hear them bog down a little bit and then go up a few hundred and just keep gonig back and forth, its real noticeable. its idleing just under 1000, id say about 6 or 700 - 900 when it varies. any ideas?

 

Check Engine light? if so, what are the codes?
Not entirely sure otherwise, but I have a couple ideas. Is the battery and charging system good? are all your battery terminals and grounds clean and tight? Vacuum leaks do strange things to idle, might take a look around for some split hoses. I had the same problem and redid the head on my t5 at 175k(different reason for that), on my way in I noticed every piece of rubber in the car was garbage. I replaced all my vacuum and pcv lines (among other things) When I finished and started the car, I had a good steady idle. WD-40 usually raises idle if it's sprayed over a vacuum leak, perhaps you could try that to test for leaks.

 

Take it to an independent mechanic or a dealer who can scan the sensors individually and tell them to check the mass air flow sensor. This sounds like the problem I had with my 850. It would go up and down when idling but be fine when it got up to speed. It took a month and $800 to figure out that it was the sensor. [:'(]

 

That's a good point. I had another vehicle that when the maf went out it would crank but not start. On most any car I've ever owned if you suspect the maf is bad, you can unplug it before starting. If it starts and idles fine, it's the maf. Bad thing is you can't drive it anywhere with the maf unplugged. Would be too cheap of a solution[8D]