1995 Buick Roadmaster

I have a 1995 buick riviera 3.8l s/c with a 4t60e tranaxle transmission. This car is great except for my one problem, sometimes it won't go into overdrive and stays in 3rd.I had it to gm who scanned it and had no codes come up.I drive alot of highway miles when it works it seems to stay working, slowing down for towns, corners etc. Fluid is clean and up. When it wont go in sometimes it will seem to go in half-way dropping from 2500rpm in 3rd to 2000 but not to 1600rpm where it should be and usually kicks itself back into 3rd(2500rpm). But it DOES NOT slip when in any gear.I don't mind the bill if I can kinda pinpoint the problem because gm doesn't know what to check.I am capable of most repairs but I can't even seem to order a book to find out how to check things such as t.P.S or the tcc with an ohm meter some info/help would be appreciated thanks!