Dug, when you say that the fuses are OK, I'm assuming your speaking of the Overvoltage Protection Relay. The Fuel Pump Relay doesn't have a fuse. I too own an '85 190e and the exact thing occured. It would stall out and hesitate, then run great for a week or so. Only after replacing the Fuel Pump Relay did the problem go away. The cheapest place to buy one is at Fastlane. I've checked around. If any type of moisture penetrates the seal of the relay it does act up.
As for the size of the alternator, go to the Fastlane site and give them your VIN and they can tell you exactly the alternator that goes with your car. (No I have no connection with this site, but have been impressed with their prices and service).