I'm hoping someone will be able to help me figure what my problem could be.

I have an '84 Peavey Horizon II guitar. Here's the pic of it at the end of the post...

It seems to have developed an intermittent problem. Sometimes it makes a hum type of noise, kind of like a gound lift switch needs to be turned on or something. It's worse when I have any of the pickups configured in a single coil mode. The noise lessens when I have the neck and/or bridge pickups in a humbucking mode and have the middle single coil turned off.

Now, here's the strange part ... lay the guitar flat on its back ... and the noise goes away completely. Return the guitar to a vertical (playing) position and the noise is there. AND ... this noise pops up for 1 to 3 days at a time, and then goes away and doesn't come back for 2 or 3 or 4 weeks.

My guitar fix it place can't makes heads or tails of it. It's definitely a problem inside my guitar though. Anybody ever heard of this??? It doesn't happen very frequently, but when it does, it drives me nuts.

Any comments or help would be greatly appreciated. Thanks.