Stress on an old manifold could do this. The exhaust system being held under the cab, and putting stress on the manifold as the engine motions. Its common. Is the manifold cracked? Or is it a leaky gasket? If its cracked, or even if its a gasket, depending on the last time the manifold was taken off, it could be a Biznatch to deal with. Due to placement, and the corroded bolts that are holding it on (unless the fasteners are regularly cleaned). Be sure to go easy on them, so you wont break them. wd-40 works nice. But if it hasnt been long, its a breeze, wont be hard ( or at least for me it wasnt), as long as you can get your wrench to the bolts. I've noticed its a lot easier on the newer F1's 04-08. As for getting one that will last, I guess the only other thing to do is switch to aftermarket, such as gibson, or hooker, for example.