2000 Dodge Ram recuring shift problem

I bought my truck 4 months ago and it started having shifting problems. The mechanic replaced the stuck govenor and the trans filter and filled the tranny with full synthetic. it worked great for a while, but now the problem has resurfaced a few weeks later. the problem is , the tranny doesn't want to shift out of first, no loss of power. i can some times make it shift by raising the rpms to 3000 and leting off the gas and shifting to neutral then back to drive. after it finnally shifts it shifts hard to second and is a little slow shifting to other gears but all the others work.then while i'm going down the road at constant speed sometimes it drops out of gear and back to first it doesn't matter if i'm going 30 or 55 and its getting worse every day and it's worse once the tranny gets to operating temp, but it never over heats. i live in new york and it's ussually betwenn 0 and 20 degrees f outside, oh and my warrenty just expired so i don't know if the dealer will fix it again even though they didn't really fix the problem the fist time just made it go away for a few weeks. please tell me what the problem is.