This is my first post, so be gentle. I have a 78 Panther that has a strange exhaust issue. At idle everything looks good, however when I rev the engine, the muffler acts like it is going to blow off the manifold pipe. I have put all new springs (flange and muffler to the base plate underneath), and still have the problem. As far as I can tell, there is no blockage. I can blow air through the muffler, and there doesn't seem to be any restriction. Any suggestions would be greatly appreciated.