What kind of ignition are you using? If you have an amplifier box (ie. MSD or similar) and it is mounted inside the cab you could have trouble. Just running shielded cable will not fix this. You may have to build a small metal baox over the ignition box to isolate the noise. You can test this by covering the box with several layers aluminum foil. Try to make sure the aluminum grounds somewhere. This happens periodically with ignition amplifiers mounted in the drivers compartment. Hope it helps.
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