We had seen some problems after flushing tranys out. Recently our dealership switched over to Wynn's products and they have a awesome transmission flush service, and we havent really had a problem since. It's possible with the amount of miles and if its never been rebuilt it may need an overhaul, those 604's are not the most dependable transaxle on the market. Another thing is that those units need the correct fluid or you'll get some slippage. ATF+3 was used then, it wouldnt hurt to try the atf+4 thats available now.