Don't add any earth / ground wires to the heatsinks
+1
The tab on FETs and other drivers is tied to the center pin of the semiconductor. You can isolate that from the heatsink or let the heatsink "float" as they have. Usually, they isolate the tab from the sink electrically, but not thermally. Little "rubber" mats and plastic washers. If the mats and washers are there, one has failed (usually from over-tightening, usually the washer fails). I probably have some if you need them, just post a pic of the FET and I'll pop a few in an envelop.
Damn, you're about out of machines, I'm guessing. If you didn't go to metal gears, you might want to order a spare gear or two, if you haven't already. I blew two out in ten minutes one fine day.
Wen I was young, I spent most of my money on fast women, slow horses, and cheap booze. The rest of it I just wasted.