1990 Pontiac Bonneville wont shift

I have a 1990 Pontiac Bonneville with the ever so reliable 3.8 liter V-6. The engine is great but the trans isnt so. When the car sits for several hours and I go to leave in it, the trans wont shift properly. It goes reverse just fine as well as 1st and sometimes 2nd gear, but it will not shift up. This is a big problem for me especially because I make emergency medical and fire responses.
I talked to a dealer who says I should be able to start up and idol for about a minute and go. I changed the filter and gasket, because I was told this is the problem. Guess what it still does the same thing. No it is not leaking, and it is sucking the gas. I was getting almost 20 miles to the gallon in the city before these issues, and about 25 to 30 on the hwy. Now I get a lot less.
Do you think changing the trans control solenoid or the tourque converter clutch lock up solenoid or any other componets help this problem.
Oh, and I also changed the modulator.
Thank you for your time.