radio signal comes and goes

[John black]

I have an 85 k5 it had factory windshield antenna but the stuff came lose from the windshield so I had added and antenna from a 94 Chevy I had I mounted it to the inside edge of front bumper on passenger side next to fender I have newish ( 6 mo. ) Pioneer radio new cables in between just replaced the antenna. ..same mount some times signal is perfectly clear fer minutes later no signal can't figure it out thanks for reading