Transmission downshifts at random

I've already posted it elsewhere on this forum and got a little feedback that was helpful. But my transmission will randomely downshift from 3rd to 2nd. It's a 350 with a shift kit and it worked perfectly fine about a month ago (Shift kit was put in ages ago by the guy we bought it from) We cleaned the modulator and put a new transmission filter in and replaced the fluid but it still downshifts randomely and then refuses to upshift until I manually downshift into 2nd.(Should probably have mentioned this is a '76 camaro with automatic trans.) I am going to spend my afternoon today getting the govenor out because I was told it could be dirty or going bad. It also only downshifts when the engine is warm (Usually about 40 minutes of driving, sometimes less) Please help as I am new to the rebuilding a car scene and want to get this thing running flawlessy before winter sets in.

First thing ... you 'cleaned' the modulator ... what does this mean? A lot of modulators have a screw inside of the vacuum nipple that you can adjust to either firm up or soften the shift, it will also affect shift points. There is also the detent cable which attaches to the throttle on the carburetor - is this properly adjusted?