Sorry to bring back an old thread but my 98 ob does the same thing. P1507 IACV fail safe, 1540 speed sensor. I did a 4eat to 5MT swap using a completely different engine. IT DID NOT SOLVE ANYTHING! So far the only thing left is speed sensor (used from previous 4eat). Just replaced that and it still does the same thing. MAF, replaced, still does the same thing. It's not the neutral switch or ANYTHING on the engine. The only thing that I havent replaced is the cat/AF/o2 sensor, and the engine computer. Idk if there's a bad connection somewhere or what..... But I'm pretty stumped.