[Soozie32]

Since November, our 1999 Chev. Astro 2WD has had a problem: It will sporadically run rough for a few seconds at highway speed. It will feel like driving against a strong wind, or driving over a lot of cracks in the pavement, sort of a bucking. Then it smoothes out and the "Service Engine Soon" light will come on. The light will stay on for a day or two, then go out. This happens sporadically, I can't determine any specific circumstances when the problem happens. It happens when I use cruise control and when I don’t; it happens up hill and down hill; it happens in wet and dry conditions, etc. The only condition that I have figured out is that it does it between 60-70 mph.
Our mechanic hooks up his scope, and all it says is "random misfire." He says that all 6 cylinders are misfiring, almost like it is running lean. He is stumped, he says something is failing, but he can’t figure out what. What we know it is not: fuel injector, fuel pump, EGR valve, fuel filter, because we have had all of those things checked or replaced in the 5 years we have owned it.
At one point this winter it refused to start, and the mechanic determined that the coil was arcing, so that was replaced. Again, it randomly misfired after a couple of days.
Then we suddenly lost all the coolant, and discovered that the intake manifold gasket leaked. That was fixed, and a new serpentine belt was installed.
Last month, while cruising down the highway, the engine suddenly lost power, dropped down to less than 30 mph, and became very loud. We had it towed to our mechanic, who said that something in the lower part of the engine broke. The engine had about 124,500 miles on it. We had a remanufactured engine (with 100,000 mile warranty), new oil coolant lines and water pump installed.
Don't ask why we are spending so much on the dumb thing. We like the seating and towing capacity of the van. Plus, we can't afford a new auto loan.
We thought that would be then end of the trouble. But again, it ran for a few days just fine, and then the misfiring started again.
HELP! What could be wrong?