1983 Chevy Silverado shift issues (search completed)

[Cochise]

Good Afternoon, long time poster, first time reader. My transmission shifted terrible from 2nd to 3rd on my 1 hour trip home when I bought my truck, also my tranny leaked like a siv. I went ahead and replaced the front and rear seals and pan gasket thinking I'd be able to solve the problem of me overheating the tranny because of leakage. I thought if I fixed the leaks, the tranny would not run hot and I wouldn't lose 3rd gear. I did all of that and it feels like it slipped, so I thought, replace the modulator valve. I did that and it still did the same thing as before. I read about turning the set screw in the modulator valve a certain increment in certain directions, STILL with no change. IT WILL kick down for me, but with regular driving the rpm climbs and the tranny slips pretty bad. It shifts fine from 1-2, so I think it's something else. Does anybody have any ideas? Also, I still have a tranny leak from the speedo cable. I put silicone around where the "O" ring is thinking the leak would stop, but it hasn't What is that part called so I can get it from the parts house?