Engine Misfire?

I have a 2002 Buick Rendezvous. When I drive it on a hot 100 degree day after about 30 minutes (nothing happens before at least 25miles), it does a hickup and the RPMs drop for a second. It is worse if I am going up hill or have the a/c on. It didn't show up under the dealer computer, so they don't know what causes it. Most recently it occured for 10 minutes jerking about (rpms bouncing) and stalled. It started up right away. The dealer let it sit for the weekend and tried it Monday driving 60 miles on a flat road and only got it to do it for a second. The scenario: It has to be a hot engine (I assume by the 30 minutes driving), driving uphill and a/c seem to get it going and make it worse. Driving on a flat road or for short trips doesn't make it happen, and mph doesn't seem to be a factor. I am picking it up today and have no idea what the problem is. Please help!

What size engine? How many miles on it? Could be as simples as new plugs and wires. A misfire should "techinical" registers a code. I'm suprised it hasn't. My thinking is driving up hill with the a/c requires the most out of an engine. If you engine is already low on power for some reason than that is going to put it over the edge.