Aside from what Mr 50 said, this is all i can think of.
Try checking your grounds. Check the ground from the battery to the engine and also the chassis ground (can't think offhand where it is in your car). My '84 had the negative wire exposed and grounded a couple inches off the battery, then led to the block (which was the stock setup). Have you had the cables replaced. There also could be high resistance in the cables if there was sulfuric acid corrosion coming out of the battery. Check and see if the insides of the cables show signs of sulfur.
Good luck, keep us posted!
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'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT
[This message has been edited by 84LX89GT (edited 10-31-2001).]