Okay, sorry for the long post, but I have been told this is the place to come and ask the experts about transmission problems. Let me start off by saying that I am a total noob with trannsmissions and I have probably done more harm than good so far, and I am prepared to be told that.
The vehicle is a 2WD 1982 Ford F150 with a 351W and a C6. I bought the truck for a song last summer and I knew it was going to need some work. From day one, I had to manually shift into 2nd gear and third was kind of iffy. I drove it to work and back all ladt summer and fall (about 5 mile round trip, so issues didn;t really matter). Right around Thanksgiving, I lost second and third gear completely on the way home, and first gear started slipping very badly. I limped it home and started researching my options. The fluid was dark, full of crud, and smelled burnt. I knew it wasn't worth buying a reman trans or having this rebuilt at a shop for $1500, so I ordered the super master rebuild kit from Rockauto and the Haynes Ford Auto Trans manual from Amazon. I read the whole thing cover to cover a few times, as I had never fooled with an auto trans before. I consider myself pretty mechanically inclined, and I figured worst case I would end up with a truck that still had a goofed up trans and only be out around $200 total and a bit of my time.
So I yanked the trans and disassembled it. As I had expected, the clutches were all fried, and I had to get a few additional parts from a local trans shop, like the intermediate band and some oneway roller thingy. I did my best to keep everything perfectly clean and organized, but I did cut a few corners due to working outside in Dec/Jan in 20 degree weather with 10-15 mph winds. I did not disassemble the valve body at all, I did not blow out the lines to the radiator, and I did not take the torque converter to a shop or replace it (although I DID drain it well) After about three days, I buttoned everything back up and was surprised that it ran out just fine and shifted better than it ever had. No slipping, no pause to engage gears, nothing. Only problem was that I did not change the front seal even though one came in the kit (I was bordering on delirium after 3 long days working in below freezing conditions). As a result, there was a slow leak from the front seal but that didn't bother me because I just kept it topped off.
Fast forward to a couple weeks ago... I forgot to top off the trans fluid after letting it sit for a while and made a quick blast down the highway to pick up some things. On the way back, I started noticing a little slipping in high gear that got horrible by the time I made it back home. It was about 2.5 quarts low on fluid, which I topped back off. The heat radiating from the transmission was intense. As a side note, when it cooled off and I did an inspection, the intermediate adjustment bolt had backed way out (with the locking nut still attached). I reinstalled it as it was, but have not yet rechecked the torque.
Now here is my dilemma: I have tried changing the fluid and gasket already, to no avail. The truck drives perfectly fine when it is first started up, but after about 15-20 minutes of driving, the 2-3 shift starts dragging out and it feels like 3rd gear slips when going up hills. Any ideas where I could start troubleshooting? Please don't say pull the trans again, I will sell the truck beforte that. I am just looking for a way to get a little more life out of this until I can afford something better. I'm thinking the issue is heat related, and something is causing excess heat to build up in the trans. Any ideas/input would be greatly appreciated.