Toyota Repair: 97 Corolla DX transmission ~80,000 miles, corolla dx, shift into overdrive

Question
I have noticed for the last year or so (18 months) that the transmission does not jump into the last gear as quickly as it used to as I increase speed.  This was noticable last winter (year ago) and then this latest winter it seemed somewhat worse, taking longer to jump in at speed, so it runs in the lower gear longer and engine sounds strained a bit.  In warmer weather or after the car was warmed up, it did not seem noticable.  But now, with the weather much warmer in April 2004, it is now happening in the morning (car is cold, but temps in the 40-60 range) when I did not notice it at all last spring or fall when starting up.  Changed the trans fluid at 65-70,000 (a little late), bought the car 2.25 years ago with about 55,000 miles.  Not sure if they had serviced the trans before I bought it, but tech said the fluid looked a bit dark.  No problems with the car until now and am still satisfied with performance generally.  Have changed oil at recommended intervals.  Am I heading for tran trouble or does this sound within range of reasonableness?  What's the possible fix and the cost of that?  Thanks for your help.

Regards,
Fred Mischler  

Answer
Try to find a shop or dealer that has a transmission flush machine and have the entire fluid replaced including the torque converter fluid, this may cost around $100 but it may help. Make sure the correct transfluid(ATF)is used, Dextron Mercon III.
If the problem persists have a Toyota dealer diagnose it, this transmission has a coolant temp sensor on the engine that sends a signal to the computer to let the transmission shift into overdrive after it reaches a certain temperature(engine temp)and not before.
Does the engine reach operating temperature fairly quickly or does it take a long time, does it ever get to operating temp?(about 1/2 on the gauge)
Before you do anything I mentioned above, try changing the thermostat.
I really don't think there is a problem with the transmission itself, they are very reliable and with proper care should last well into the 100,000 mile range.