The recent life story of my '85 240 DL: It started acting up shortly after filling up with cheap gas. It was raining that night, and I thought at first it might be either the rain or a bad tankful. But, for the past couple of weeks, the engine intermittently runs very rough (hesitates and surges) at any speed from idle to expressway, in various kinds of weather, on near-empty tanks and full, cold engine or warm. It hasn't stalled yet, but threatens to, and often has trouble making it up hills. It also runs inexplicably beautifully a few miles to a few days at a time, and then suddenly gets worse.

I changed the distributor cap and rotor maybe 15K miles ago, and have 3-4K miles on a new set of Bosch platinum plugs (had similar problems before changing plugs). I've got a set of spark plug wires on order from IPD, but they haven't arrived yet - will inspect plugs when I install the wires. I didn't find Techron at my local shop, but I added Gumout XTRA fuel injector cleaner when I filled up 2 days ago. No improvement evident in the first 50 miles.

I just had a new timing belt professionally installed a few days before the current problems began. I hope that's not the cause.

Any thoughts on what else to check?