Ford: Ford Windstar surging/high rpm at speed, vacuum modulator, ford windstar

Question
Hello Ken,
I have a 2000 Ford Windstar 3.8L with ~95000 mi. The problem happens on the highway, usually beginning after I accelerate out of a toll booth.  Cruising at '75 mph, the engine revs from ~2400 to ~3400 almost as if overdrive is not engaged, but is engaged because when I turn it off I notice an immediate difference.  Also if I back off the gas a little, the rpm will return to normal, but as soon as I attempt to give it more gas to keep the speed, the rpms surge.  Please let me know what you think this may be.

Thanks In Advance,
Corey

Answer
Corey,

It sounds like the overdrive in the transmission is going bad or the overdrive switch engaging and disengaging. It could also be that your time is off at that high speed, low fuel pressure (common in your vehicle at high speeds) or your vacuum modulator on the transmission could be going bad, causing the transmission to slip at high speed. Hope that this helps and if you need further assistance, please do not hesitate to contact me. Good luck.


Ken