89'lock up in 3rd gear (ocasonaly)

i have a 89 ranger xlt extended cab. i bought it off a guy that had given up on this problom. ever once and awhile, the trans. locks up in 3rd gear.and it wont come out. its a manual ,mitsubisi , 5-speed. 12 bolt pattern on pan trans. ,the way i have to get it out,is to take off the pan,and unlock . he said he had taken it to the shop,and they couldnt find any thing wrong with it. so now its on the binch at my mecanics shop, and he cant get it to lock up either, so now, im faced with puting it all back together, and waiting for the next time ,or puting a diffrent one in.it only locks up, in 3rd gear all others are fine. have you had this problom before. any ideas. need fast help!!!

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