I've got a 2004 ESV with about 75,000 miles.

At around 50 mph, the RPM's go from around 1500 to 1700 to 1500 to 1700 and so on. Any ideas on what could be causing it?

When was your last tune up? If you never had it done you want want to start there. :)

I bought it from a big and very good dealer, and they'd serviced it completely before I bought it. It runs great. The only problem is that from around 47 to 53 mph, the RPM's cycle up and down. At 60 mph, it's fine again. At 40, it's fine. Any ideas?

Is it shifting up and down between 3rd and OD. I mean is it having trouble staying in one gear. If so try driving it with the selector in 3rd instead of OD and see if that helps.

I'll try that (driving in 3rd as opposed to OD). I think that it's not actually shifting gears back and forth, but it could be. I'll let you know what I find out, and thanks for the suggestion.

Did you find the problem / Fix to this .. I have the exact same problem

TIA