I had thos probsall the time with my 3i.... But the nuts weren't loosening off cos I use 2 nuts on each bolt - Tighten down your first nut as normal, then tighten down another nut onto the first one. Two nuts tightend against each other create a locking section where neither can undo unless you get in there and give it some real force.

Anyway, the probs I had was the downpipe kept cracking and snapping off. ON the bottom of the 2 branch downpipe, where it turns to one. Right at the very end where you have the rolled lip where the system butts up against it, mine kept cracking there. So no matter how many times I checked the tighnes of the nuts and botls, it was the crack that caused it to blow like a fooker