Intermittent problem
 

So this past weekend I was at an auto-X and after one of my last runs my car's idle was off, vacuum was down and my A/F gauge was reading close full rich instead of the normal full lean. Water temp was normal for an auto-x at about 200F and driving at speed seemed pretty normal.

I thought I popped a hose or something so looked around under the hood but found nothing. I needed to get home so that's what I did. On my way home the problem vanished.

Does anybody have a clue as to what this could be? I was thinking maybe my O2 sensor but if that were true my A/F gauge wouldn't have been able to tell me I was rich. (The back fireing confirmed it though.)