Weird Noise thru my Amp - Happens every time I play, with or without an amp!
My guess echos what jseth said, that it is heat related. I spent some time last week at Tybee Island, Georgia. The temperatures were in the 90s, but in direct sunlight things quickly absorbed a LOT more heat in mere moments. Step one, then would be to get your amp out of the direct sunlight, perhaps with reflective umbrella or even a heat reflector designed for a windshield. Do not block air flow around your amp, of course, just get it into the shade. This might not work, but it's a relatively inexpensive experiment.
cotten