Help! New Fender American Jazz, almost new Hartke 4000 amp, new Ampeg 4x10 8ohm cab. Set it up at home, sounded good, no problems (at low volume...family complains otherwise). Get to gig, set volume at 4-5 and get loud hum and intermittent clipping. Stays like that all night. Get rig home, leave settings alone, plug it in, sounds good again! Obviously electrical problems at bar, right??...or maybe not?? Problem is we got to play there another 5-6 times this summer. What can I do to fix my problem? Should I be overly concerned with the humming and clipping or not since I'm not getting the problem at home? Thanks for any help. Youse guys are great!