After removing front and rear valve bodies, checking governor valve action, kick-down solenoid, TV (cable actuated) valve on valve body, action of 1-2 shift command valve and a few others, all I found was some dirt in one of the little screens in the rear valve body (as suggested by David S Poole). Everything seemed fine otherwise. I put it all back together with new gaskets and now transmission works fine. I guess it must have been the small amount of dirt clogging the thimble-shaped screen. This has certainly taught me to be more dilligent about trans fluid changes.

Thankyou all for the helpful suggestions!