1997 GMC C1500 6 Distributor Caps in 2.5 years!

Local shop has replaced my distributor cap 6 times over the past 2.5 years! Everything is okay when new cap installed but after a few months, the truck seems to "skip" or "miss" when driving, causing the truck to jerk a bit over and over - kinda like the gas isn't getting smoothly where it has to go. I have a manual transmission and it happens in 1st to 5th gears, especially when accelerating.
This problem seems to start initially when it's damp/wet/humid outside, but then get progressively worse over a couple of weeks until eventually it won't start at all. Shop replaces distrubutor cap and all is well again. Why does this keep happening? Thanks!