2005 Honda Odyssey Trouble with driving long distances

Twice in the past month my Honda Odyssey after driving for over an hour the RPMS go sky high and car only goes up to 5 miles an hour. The car can not go into reverse and steering is difficult. The air conditioning was on during both of these events.
Upon having it towed to the Honda dealer both times, they could not find anything wrong with the car, it runs fine.
They ran diagnostics both times and found nothing. The check engine light has never come on.
The AC runs hot air until I put the temp as low as possible. Then I can adjust temperature.
Does anyone have any idea what I should do. I have had all of the work done on the car at the same Honda dealership since purchase in 2005.

You obviously have a transmission shift problem.

Take it to a good indepoendent Transmission shop and hope for the best. Have you checked the fluid level? How many miles on the car and has the fluid been changed and at what mileage. The Honda manual may call for a fluid change, but it is strongly recommended.

I wonder if for some reason the brakes are locking up. If this is the case, the transmission would be pulling in the lowest gear to overcome the brakes. I had a similar problem with a 1990 Ford Aerostar. This Aerostar only had anti-lock brakes on the rear wheels. The Ford dealer couldn’t find the problem, but a mechanic at my independent tire shop traced it down to a gummed up valve in the anti-lock brake system.
As far as the air conditioning is concerned, this is probably a separate issue.

The car has 140,000 miles on it. The car is steal at the dealership. I called and told him both of your responses so they are going to check the brakes. He said transmission fluid is clean. They are driving it around right now trying to recreate the problem. They are baffled. I will take it to the independant transmission shop once I get it back from Honda. Thank you both for your responses, they were very helpful!

Has the recall been done on your van?