I would suggest making sure that you have ground straps in place. I had a 69 GMC I was working on and getting all sorts of weird things happening. Someone on here suggested to me that I make sure I have ground straps in place. I crawled underneath and some of them were broken, some were missing and things worked a lot better after I had suffecient grounding.
Here is a post from the FAQ section showing were they are supposed to be located.
http://67-72chevytrucks.com/vboard/s...ghlight=ground