2003 Pontiac Montana Fast Idle

When the outside air temp goes above 50 degrees the van will start to idle high. Normal idle is about 600 when it is below 50 jumps to 1200-1500 over 50 degrees. Started doing this over a year ago. Took in for service last summer and the mechanic could find nothing wrong. Have done nothing over the winter since it quit acting up. Today is one of the first days that the air temp is warming up the problem reappeared. I have also noticed the rpms at the transmission shift points goes up to over 3000 before the transmission will shift.