Annoying Suburban problems

[skyhawk1]

My 72 Suburban is my daily driver. Rides like a Cadillac. Here's the main problem. I put a HEI distributor in it in the Spring this year and it started great for about 6 months. Now, just about every morning it cranks but won't start. So I get out, raise the hood and tap on top of the coil with a small screwdriver handle (usually 2 taps) and it fires right up and runs and starts good all day. I've replaced the rotor button, dist. cap and even the ignition module once. The last time I replaced these parts it started good for about a month, now it's doing it again. Any suggestions?