I would try another location to hunt and if you can get your hands on another coil you could try that. When you said it did the same on your 6x probe, that to me would be the same as changing a coil. But what you described is the very, very same thing mine was doing. I bought mine back in the winter (used) and all I could do was some air testing here at the house. Never even knew until I took it out the fist time several months later that I had problems. I started off at a small park and worked with it for several hours, falsing on everything, then I meet up with my brothers at an old school yard, worked it there, let my brother try it, changed the coil, no luck. I had to get a backup F75 out to finish the day. What ever you do air testing around the house will only make you think, maybe its not broke.
I sure hope you can get it worked out, but from my seat here, it didn't sound good.
Good luck with your testing and let us know how it turns out, we and I for sure feel your pain.
Go down about 17 post below yours and read through mine about 1270 coil air test, see if that sounds like the same problem to a T.
Ron in WV