Ocasionally an exhaust leak can cause this problem, that fresh air is drawn in and reignites spent fuel not fully burned during combustion. This is more common with headers, but a cracked manifold can do it also. In some cases a sticking valve can be the culprit. In addition an intake leak can cause the problem.

Easy way to check for ignition leak is to use an unlit propane torch with the gas valve cracked open, and play the gas around mating surfaces. When rpm's increase you've found a leak. There may be several areas leaking, so don't stop checking if one is found.