Toyota Repair: engine revs hgih without acceleration, sudden acceleration, air conditioner condensor

Question
Hi Vince, our 1998 sienna has been acting up.Recently it has an infrequent but a curious issue. The engine revs high to about 4000 rpm form a stopped position with light peddle pressure. the vehicle does not jerk or shudder and shows no sudden acceleration on the speedometer, just high tach rpms and a loud audible squeal. We had both engine belts replaced thinking they may be slipping due to age and the mechanic said that the air conditioner condensor may have contributed by having water drip onto the belts. the weather has been very humid lately, but to no avail the problem came back. Could a faulty maf sensor cause this or slipping belts within the transmission. I was also wondering if a damaged cv joint could be the cause by spinning if it is worn?

Answer
My '97 Avalon did the same thing; unfortunately for you Mr. Transmission is ready to die.  There are bands inside the transmission as well as numerous little passages for the transmission fluid.  In these passages are springs and small rubber valves.  Yours are probably worn out and not directing transmission fluid to the appropriate place at the desired time, hence the revving without moving.  
A Band-aid approach would be to change the transmission filter after flushing the old fluid out.  This may rejuvenate the seals and remove debris from the system.  BUT, this is a TEMPORARY fix (e.g. a couple of months)
Find a good, trustworthy mechanic who can locate a low mileage used transmission to replace this one.  Due to the age of your vehicle you probably do not want to spend the $$ on a new or rebuilt transmission.
btw, while your mechanic is changing out your transmission, he may recommend replacing your engines' Rear Main Seal.  Take him up on his offer.  To change a rear main seal necessitates removing the transmission; so, since you are already there...

Hope this helps.

Vince