Volvo: 1990 240DL automatic transmission problem, automatic transmission problem, last ditch effort

Question
Ray,
When we put our volvo in drive, sometimes it shifts just fine and other times it doesn't want to shift from 2nd to 3rd gear.  One time recently while driving 55mph over a 4 mile stretch it shifted on it's own from 3rd to 2nd.  If you shift it manually, it seems to work fine.  Would this be in the transmission, or is it a sensor?  

Answer
James,
Most times that the Trans "holds" the shifts too long I find it better to replace the Trans with a good used one. However as a last ditch effort you might want to have the Trans fluid flushed and see if that will afford you any better shifting. Flushing you add a cleaner and then completely remove all the fluid flushing it completely while adding new fluid. That process removes the old fluid and varnish that builds after time.