First make sure that the cylinder isn't cracked near the exhaust stud areas. This is rare, but it does happen.
Next thing I'd do is to check for a clogged exhaust system (especially if you're running an EPA approved catalytic converter pipe). You can search this forum for more info on how to unclog the exh. A quick test would be to remove the exh. system. A very short drive should be enough to test it (don't go around blasting everyone's ears out).
Also consider anything else that you might have done to your bike (besides tightening the exh.) shortly before this problem began (i.e. filling the gas tank, spark plug change, tuning, mods, etc, etc).
Btw, it'd be good idea to change the cheap Chinese exh. studs, nuts, and washers out for higher quality units (you can match-up replacements at your local hardware store (i.e. Ace, True Value, Home Depot, etc). You can also order these, and other studs, nuts, and fasteners in general from Custom Motored Bicycles, That's Dax, and many other outfits.