Any ideas on what would cause that? We have had some storms and really hot and humid weather around here lately... Perhaps it is interfering with something?
Seem like interference of some sort. You mentioned the "wire" run is long - is it coaxial or 300 ohm twin lead? The 5 minutes seems like an awfully long time for something to be happening... how's your cable route? Close to a washing machine, or something with a motor? Is the coaxial cable properly grounded? Are you using quality cables (RG-6, or RG6 Quad Sheild)? Maybe your amp is faulty, overheating? Does it happen on all RF channels or just certain ones?
Check on these things...