I know extactly what you mean. Sometimes that melody you carry with you is not so much a "melody" as it is a rythmn, or a chord sequence, or, as you say, just a feeling. I have found, over the years, that if I play around with that basic "melody" on the guitar, and start to experiment with all the possible chords, it can grow and evolve into something else, without necessarily losing the flavor of the original. You know, try the relative minors, work with the obvious bass lines, get into some chromatic runs, etc. The next problem is that once this "new tune" has evolved, you now have to get of that one! Good thoughts. Bob