2000 Accord won't back up

I have a 2000 Accord which, sometimes, when it’s hot, doesn’t want to back up. What happens is that it starts backing, then it’s like someone puts the brakes on. The engine is straining, but the car is hardly moving. A shift into Neutral frees it up, and going again into Reverse repeats the process. I changed the fluid but that didn’t help. The service guy at the dealer said he’s never heard of this and would like to see it. But it only happens occasionally, so I can’t demonstrate it. Any ideas?

It’s not a V6 is it? If so, you may still be under warranty. Honda extended the transmission warranty on those cars due to known problems.