2000 Chevrolet Tahoe Idles high

Ok so normally my truck runs good but for some reason when its hot and I drive it for long periods it begins to idle high. When it idles high it starts at 1000 rpm then goes up to 2 or 3. But the funny thing is when it idles this way the gears wont change so I have to take my foot off the gas in order for the gears to change like a stick shift. Then when I get to my destination I put it in park and then it idles like I have my foot on the gas. Sometimes I feel like I have to put more pressure on the breaks because I feel like it wont stop. I really love this truck but I'm getting frustrated because no one can tell me what to do. My mechanic said he doesn't know what could cause that unless it does it for him. But I'm not taking it to the mechanic every time this happens because once I get it there I am sure it wont do it. Please help me thanks