Hi Ronnie,
I run the F75 which is the same manufacturer and Grubstake is definitely guiding you in the right direction here.
Ive never found anywhere I can run my sensitivity (gain) all the way up to 100 on mine or it will go nuts too. Ill settle for 80 at the top end, and 40 at the lower.
All things being equal get a coin sized target and test your air depth, Im sure you will find even at quite low levels of gain youll get a very good result. At 40 on mine the difference in depth would be less than 20% between flat out at 80. Usability might not mean a huge decrease in depth or sensitivity relatively speaking.
If its anything like my fisher it will also show chatter with mild emi, and the only tool in the box youll have is to adjust gain down. Sometimes changing the direction (walking away from source) will also help.
I would suggest turning the gain up as high as you can and coming back to the number just a couple below where the occasional number is falsing through. This will be the optimal gain setting. Then adjust your threshold from the - toward the +, to give you the threshold you want. On mine I neg it out until it just breaks into a sound which is generally -1, but Im chasing coins.
If your gain is too high not only will it become horrible to use, the target ID will drift up. I think all fishers have a bit extra under the hood, but more like a big block carby fed chevy than a fuel injected sports car. You just got massage it to something tame enough to use properly.
I used plumbers tape on my coil screw to get it solid.
I have no idea why the little coil is so erratic, seems odd to me.
Best of luck to you, I hope this is a little helpful.
Cheers.