I think Twisted is right on both counts. You maybe running a bit lean, and getting some backfire that is not quite audible. Enough to blow a loose fitting cap off. Can you rev the engine when it is parked and blow the cap off? If not try retarding the timing a few degrees and see if it still does it. I have that problem with the carb on my 350, but none of the caps are loose. 8-]

btw What do the plugs look like? real clean?

Larry

Ignorance can be fixed Stupidity is forever