[KiltedBanshees93GT]

I'm no paint guru, but what it sounds like to me is that the original rattlecan paint is incompatible with the stuff you're using now. you said that you were using an epoxy primer at least for the second and third base coat, is that what you used for the first coat as well?
What kind of spray paint did you use for the old paint?
Basically the purpose of primer is to "grip" the piece painted and then give something for the paint to adhere to. It sounds like the primer isnt able to grip the old paint properly, causing the crackle effect as it dries.
Also, any type of residue left on the piece can cause the primer to loose adhesion. Were the parts that are giving you trouble frequently subjected to oil/gas or other fluid spills?
If I were you I would carefully wet sand the piece down to the original primer. Once youre down to the primer, I would wash the piece with a mild dish soap to make sure all oils etc are removed, and then air dry the piece, and and re shoot it.
Hope this helps.