Here's what I'd do:

1) Check to see that the "spring" bolts at the header flange are torqued to spec (16 lb.ft) and that the springs are not bent or failing. Also, when you torque the spring bolts, alternate sides, so the gasket is compressed evenly. According to the Helm's manual, they recommend replacing the nuts...so they may be backing off.

2) I'd see if the flange on the cat is still sound. If you're in the snow belt, I wouldn't be surprised if the flange is failing due to rust. h:

3) Be sure the donut gasket is the correct size. I know this sounds rediculous, but when I swapped the old gasket onto my header, it was a very, very tight fit. A generic gasket from Kragen may not be seating properly.

Good luck.

And BTW, welcome to HAN.