1994 Mazda 626 Usually Starts O.K; Trouble Sometimes Resta

This is an intermittent starting problem (once every few days). If the engine stalls (for example when I don’t give it enough gas when letting out the clutch), it sometimes will not restart (turning over, but not starting). Once or twice it cut out on its own. When cranking, I get a bit of gasoline odor that suggests that gasoline is getting into the cylinders, but it appears that there is either no ignition or flooding. If I let it sit for a while (5-10 minutes), then it will restart. Once restarted, it runs fine (though a little rougher the past couple of days). It seems to be more likely to restart if I turn the key completely off before trying to restart (suggesting an electrical intermittent like a relay, or a cycling problem with a pump). It seems to be more of a problem to restart after the engine has been running (warm?), But has happened once on a cold start attempt as well. It is not the idle air control valve, which I replaced with no affect on the problem since the problem started about a month ago. (The IAC was replaced because of the more typical problem of it sticking closed and killing the engine idle. That was a problem once every couple of months for a year, and easily corrected with a tap on the IAC body.) The Check Engine light has NOT illuminated. Update: After a month of the intermittent problem the CEL light did come on today. The codes are 15 and 17 (Haynes says: "oxygen sensor or circuit" and "oxygen sensor (check the fuel and ignition system performance)".) But it strikes me that these codes may be symptoms (because of the unburned fuel) rather than the problem. Thoughts?