Try recalibrating the unit. Make sure to have no cables in the tempest. I used to have this problem. It was so long ago though I forget what it was.
Reinstall the OS's. First an fore most. Reinstall it all.
Then Try recalibrating the the OSC
Then Recalibrate the Drum pads.
To try an isolate it if its a bad voice especially if you find you hit a pad in sequence an every time its the 5th hit. Your 5th voice might be going. Go to the voice assign page an hit the pad going through all the voice assigns. If you hear a difference in the voice then it might be that as well, easy trouble shoot.
If it's still persisting it could be do to the pads on the tempest were screwed to tight at the factory an just need to be loosened. You will have to open the tempest which is a very easy thing to do. I honestly can't remember what it is. Usually though this would cause pads to trigger other pads.
Contact DSI an they will for sure help you.
Also to mention I had this problem. It seemed to fix it self. Earlier this year after having the tempest since its release an using nearly everyday. The main board with the Voices went on it. I think voice number 4. Very easy. DSI sent a new board, I opened it up, replaced it an it sounds just amazing, a placebo effect of better then before. Warranty is only $25 USD, you just have to pay $150 deposit they refund when they receive your old board. Just a thought. Great service.