I have a 91 Pathfinder, 2WD, manual trans, with somewhere around 350,000 miles on it (we're not exactly sure because it was driven for a while with a broken speedometer). It has been a great truck, but recently it has developed a problem I can't diagnose.

Intermittently, the idle will just drop to 400-500 and surge, threatening to die and then recovering, and once in a while it will actually stall. It starts right back up, and sometimes the problem will go away after a restart and sometimes not. It can happen anytime, but it's most noticeable after it warms up and drops to low idle. The CEL has come on once or twice, but I couldn't pull a code. Normally it idles around 900 and it runs just fine.

It seems to have started doing this since the weather turned cold, but the truck was parked for a month or two before that and I had to charge the battery to get it to start. I'm not sure if I fried a sensor or something, or if it's related to the cold weather, or what. Any help would be appreciated; it's due for an emissions test very soon.

I would suspect the airflow meter.