Volvo Repair: Transmission not shifting into OD, transmission shop, trans fluid

Question
my '91 940 GLE won't shift, gets stuck & won't let the overdrive kick in. running the engine @ a high rpm, while going going down the highway..kicks in after about 10 miles.  Replaced the OD relay - flushed tranny fluid & problem has not improved....  It only seems to work after it has been revving high for a while, any idea?
Thx for the reply...

Answer
Hello,
         The fact that it kicks in means that it is basically a working unit...however you might need a tranny expert to look see...be certain that FLUID LEVEL is proper...often when trans fluid is neglected and it "Gums up" the insides of the tranny..it has shifting problems....be certain you use proper type tranny fluid....there are at least 4 types sold....if I were you...I'd put a high quality tranny suppliment additive like Lucas trans oil...its worth a try....Read the instructions.
         BE VERY CAREFULL IF YOU CHOOSE TO BRING YOUR CAR TO A FRANCHISE TRANSMISSION SHOP...YOU COULD BE ROYALLY SCREWED....AS MANY OTHERS HAVE...I'VE HEARD THESE STORIES..FIRST HAND....get at least 2 estimates

            Good luck,
                    Bilbo