Transmissions keep blowing third.

[c10Hooligan]

I have a 1985 Chevrolet C10 2wd with a 700r4 Transmission. For some reason third gear seems to keep going out on all the 700r4's that I put in the truck.(I have gone through three so far and am on my fourth) I think it might either be the TCC or the Computer but I'm not sure where to go from there. I tried to order a new computer for the truck but the computer that came in was twice as big as mine and had two spots for plugs rather than one spot like mine has. Is there a way to test the lockout switch? or does anyone have any other ideas as to why the transmission would keep blowing third gear out? Thanks for you're time.