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The easiest thing to do is the shifter alignment. If it continues popping out of gear after that, then its time to contact RPM Transmissions or similar for some re-work of the tranny. Since he's got a '99, one slight complication is that it is possible that he got a BorgWarner transmission from the factory. My '99 did also but none were supposed to get those old ones. If he did get one as I did, hopefully he did not allow anyone to put in Mobil1 trans fluid into it like I did too mine... it cost me dearly.