1999 Mitsubishi Montero idles too high

1999 Mitsubishi Montero mileage: 109,210. My truck was working fine but due for a major tune up. I took my truck to a local shop for a tune up. When I picked up my truck it drove fine for about twenty minutes, but when I came to a stop I noticed my truck's idle was very high and fluctuating from 2500 to 3500 RPMs. The idle should be between 500 and 1500 RPMs ideal is at 750 according to my manual. I took it back to them and they adjusted the throttle. I drove off again and in twenty minutes my idle was at 4500 RPMs. I found a mechanic at 6:30 PM on Saturday, Memorial day weekend, he said the distributor was anywhere from 45 to 90 degrees out of position and for the adjustments they made to my throttle they should have been fired. I brought my truck back in today and again they did the timing and adjusted the throttle. They hooked it up to the diagnostic and nothing came up as a problem. After thirty minutes of driving it out of the shop my truck's idle was back to fluctuating between 2500 and 3500 RPMs. Any ides what they could have done during the major tune up to cause this? As the problem was not there before the major tune up? Vacuum hose? Throttle sensor? It has to be something that builds up pressure since it only starts acting up after twenty to thirty minutes of driving. I am baffled I really hope you can help.