Volvo Repair: Volvo overdrive problem, volvo 740, partsman

Question
I have a 1991 Volvo 740 wagon that has trouble going into overdrive.  It eventually does shift but often waits ten minutes or longer, even while on the interstate.  It shifts smoothly between gears, including overdrive once it finally shifts.  I replaced the overdrive relay and had the transmission serviced, neither helped.  Fluid level is good and weather doesn't seem to affect it.  The car has 150k miles and is otherwise in great shape.  I do most of my own work and feel pretty mechanically inclined.  Thanks for the help.

Answer
Dear Jacob,
             You did not say if the trans is auto or standard...I'll assume it's automatic.
             It is critical to put the RIGHT type of trans fluid...there are over 4 kinds.Look at owners manual to know for sure.DO NOT LISTEN TO PARTSMAN WHO SAY.."THIS WILL WORK".
VOLVO AUTOMTIC TRANSMISSIONS ARE DELICATE.
            The first thought that comes to mind...not enough tranny fluid...
             next choice...trans shift cable..this has some effect on shifting from one gear to next.
             next choice...tranny has sludge factor...which means...dirty fluid left in trans too long left deposits on surfaces that should move very easily.A high quality tranny additive/cleaner added...then changed as directed.
          next choice.. Overdrive relay faulty.    
              next choice...the tranny Gods have issues with your car.
              You may have parked too close to one of those Korean imports and picked up something thru your tailpipe
                  Good luck,
                          Bilbo