Strange electrical problem with EGTs

Wired in my Koso gauges and took the power from what I thought was an unused acc. plug behind the plate. Unloaded and started it. It would load up and run on one then die. Changed plugs bla bla bla. Finally cut my power line to the EGTs. Sled an great all afternoon. Was goint to redo my power line today. Unplugged my revers beeper and checked the plugs to see which was hot. When the tester touches the power lead it starts to kill the engine? Could someone please shed some light.

I'll look for it in the morning. Thanks.

Just in case there are any other cone heads out there, ground to the block! Works perfectly, forgot the engine is rubber mounted.