I have a 1990 jeep yj. I swaped a 1989 chev 350 tbi motor out of a suburban. It was in for about 4 years and ran normal til it got a rod knock. So I put a gm crate motor into it. Now the problems begin. It runs fine at idle til it goes into closed loop. The timing is set at 10 degrees. After 5-10 min it starts running rough and dies. Its advancing the timing on its own. Just before it dies the timing has reached about 37 degrees. Stumped. Any thoughts?