High engine idle when driving and shifting gears

I own a 96 GMC Sonoma 4.3 Engine. 426986 Miles. I just had some major problems with starting which turned out to be timing chain related. Now that's fixed, but I am having a real strange time with idle of the engine. While driving and shifting gears I notice that the tachometer is showing the engine idle sticking around 2,000 rpm's. Could this be the Idle air control valve? Some days it does it only once, but today a short trip less than 10 miles, it started and continued the entire day. What's odd is that when the vehicle comes to a complete stop, the idle will drop to a almost normal range. But the vehicle when stopped before it drops will hold the idle around 2,000 to 1,500. Help me please?