That sounds like a bad ground some where...clean your batter cables/posts and check any chassis grounds you can find...clean 'em up and see where you stand.
New Toy- Stone stock '88 T-bird Sport in need of some TLC
1988 Cougar XR-7- HO/T-5 conversion, cracked block prevented from reaching full potential.
RIP '88 T-Bird....firstname.lastname@example.org...best sleeper in So. Jersey, now it's taking a permanent dirt nap.