u could be sucking melted snow through the intake we all know water is not a good thing. heres one more little trick pull the distributor cap off and check to see if the little screw that holds the rotor in place has fallin out. "i have seen this happen. and last but not least we serviced a mini van that was having battery problems but held a charge every time we later diagnosed that one of the cells must have a short in it that when vechicle was moving it was shorting out later replaced battery and it ran perfectly ever since.
hope this helps good luck