91 Plymouth Sundance Problems

I have a 91 Plymouth Sundance 2.2litre I bought new, now has total 258,000 mi w/45k on new remanufactured engine. This car gets driven alot! As you might imagine w/that many miles, I've replaced most everything on it except transmission. Well now it is in the shop again. It has been running great but the other day I had to drive up a steep mountain in snow. The car didn't seem to be having any trouble, temp stayed normal, no lights coming on and it was going right up. But then it started making a funny noise, which I thought at first was my chains dragging (I later found those had fallen off long before I heard the noise). But it kept going up till I got to the top where I had to stop and turn around, then I found suddenly the engine wanted to stall and I would have to keep the foot on the gas to even start it and keep it there or the motor would die as the idle speed was so low. This had never happened before, the car has always run well w/plenty of power up hills, etc, no stalling problems. The noise was a strange whooshing flapping sound that seemed to be associated w/ use of the gas pedal. And the car ran very rough. I was able to drive back down the mountain, into town and to a repair shop, but was difficult to keep it from dying at stoplights and while driving it would jerk hard frequently w/slightest change of pressure on gas pedal. Only time it would go smooth would be if I was going fast enough to let it cruise in 3d, 4th, or 5th gear and take my foot off the gas for a minute; then the noise and jerking would stop. It also occasionally belched black smoke which it never had before. Mechanics said I needed to replace the Catalytic Converter, which was making the noise, but the rough running was something else. It did need a tuneup which had not been done in quite awhile, so I'm having that done. But when they checked compression, they said I was only getting 90-100 on all 4 cylinders. At least they were even, and nothing is leaking. But they said the engine was "tired" and probably would not last much longer, even tho it has only 45,000 miles on the engine. They seem to think this was likely to happen w/ remanufactured engines. That seems incredible to me, the car runs great, plenty of power, no leaks and has run like this ever since I put the new engine in. (Actually the old engine did too, blew the head gasket on that at 212k and started losing water badly, but it still ran strong). Can anyone tell me, what is so terrible about only 90-100 compression on all 4 cylinders if they are even and the car is running good? They said it should be more like 140.