This could be due to a couple of problems but the most commen one is the layout of the wiring. the RCA cables coming from the radio to your AMP are probably routed with the 12v cable coming from your battey to the AMP so as you drive along it interfeers with the RCA cables. run the 12v cable down one side of the car and all the other cables down the other side. if this is not your problem it might be due to a faulty altenator which is overcharging and needs to be replaced. a temporary solution could be a noise filter.
read ther link below for more indepth detail on noise filters.
Answers & Comments
This could be due to a couple of problems but the most commen one is the layout of the wiring. the RCA cables coming from the radio to your AMP are probably routed with the 12v cable coming from your battey to the AMP so as you drive along it interfeers with the RCA cables. run the 12v cable down one side of the car and all the other cables down the other side. if this is not your problem it might be due to a faulty altenator which is overcharging and needs to be replaced. a temporary solution could be a noise filter.
read ther link below for more indepth detail on noise filters.
http://www.crutchfield.com/S-kWPVxOR1ggY/learn/learningcenter/car/noise_suppressors_installation_guide.html