I have a 91 Z28. It has a 383 and a stealth ram intake and tpis zzx cam. It also has a msd billet distributor, 6al and gm blaster coil. This is the problem. I set the timing by unpluging the small brown wire on the passenger side of the engine bay. Start the engine and adjust the timing to 10 degrees. When I turn the distributor to adjust the timing I get to a point when I turn it a little bit it jumps from about 0 degrees to about 15-20. But when I come back it drops back to about 0. If I leave it there and rev the engine a couple of times without letting the distributor move the timing comes up to about 6-8 degrees. I know it is not the distributor or anything else in the car because I just had a built 350 in it with trick flow heads and lt4 hot cam. Everything was fine. I had this problem with the same setup in another car. The only thing in common with both cars was the long block and the custom tune from tpis. Does anyone think it is maybe the tune? Does the knock sensor work when you unplug the brown wire and set the timing. Is the knock sensor retarding the timing while I am setting it? This is driving me crazy because I cannot set the timing right. The car is only running a 12.20 like it is and I know it has a lot more potential. Any help would be great. Thanks.