Strange issue once in a while.

It seems that once in a while ( once every outer month or so ) I'll park my 72 Demon 340 get out hang out for a few hours or so. When I get back in to start I turn the Key and nothing. Everything else works fine radio, lights ect. I'll wait a while longer, nothing else changes and it starts just fine. Like I said once every two months or so, any Ideas? I'm leaning towards the starter relay, any other opinions?