phantom powered mics and VLT

Hi, I'm trying to troubleshoot a serious problem with my live performance set up. I have experienced 4 times a sudden screeching feedback sound that comes out of nowhere in the middle of a show.I am a solo pianoman and dont move around while I sing and play. I have a great mix,and I use a top shelf Sennheiser E965 condenser mic running on the VLT's phantom power,and I bring in the harmonies via the VLT's dedicated 3 button footswitch. when the sudden screeching feedback type sound occurs, i merely shut the volumes down on the mixer,and switch off the VLT for a moment,then back on again and everything works as normal again,except for me because I'm having a heart attack and hyperventilating while trying to apologise to my audience for the shocking sudden screeching. could power surges thru the wall socket be responsible? should I stop using the VLT's phantom power and use the phantom power from the mixer? Under warranty my VLT retailer sent me a new VLT after I complained about the problem,but sadly the problem has happened again-what could possibly be happening? the Sennheiser is as new as my VLT which is only 4 months old.the problem doesnt happen every show_ please help!!!! regards Paul