Jimmy Transmission Problems

[kodyb87]

My '86 Jimmy is completely stock except for the mildly built crate motor and 33x12.50's on it. The sticker says 3.08 gears and I know it's a 700R tranny. Since I've owned it, it's only seen highway miles back and forth to work. No rocks, no mud, no abuse. About 6 months ago it stopped coming out of first gear and when it would shift, it acted like it wanted to skip straight to third. Took it to the shop and he said it was toast. Bought a new transmission and a about a month later it's doing the same thing. It blew the pan seal so I replaced that, the filter, and all of the ATF. Ran good for about 2 days and then it started acting up again. The confusing part is, some days it shifts like a brand new one. I got in it yesterday morning and had no problems driving it around the neighborhood. Yesterday afternoon when I got home from work, it wouldn't shift out of first. Any suggestions? I can't afford to keep replacing transmissions. Thanks in advance for the help.