starting problem

My 1998 Silverado, 5.7 starts hard only when it is raining or snowing after sitting unused for a day or two and only does it intermitantly, I usually get a kick out of it when I dis-engage the starter, to get it to start, I plug it in for about 3 or 4 hours, pump the gas and keep my fingers crossed. I have replaced plugs, rotor and distributor cap twice in the last 70,000 km after it starts there is no indication there was even a problem. This has happened twice in the last 2 weeks, any help would be appreciated
thanks

You plug it in? Are you saying the battery is going dead so it is slow cranking? Please clarify.

sounds like it could be the pick up coil or the module in the distributer