Never had any problem of this sort but you could try a re-sweep of the spot you have balanced over with the trigger held in so your in all metal pinpoint mode just to see if your balancing over metal or a hot or cold rock though if working correctly the detector should warn you of this. If its not that, it could be that they have checked your machine on the test bench not outside in the real world. I had another type of fault which they would not admit to. In the end I caught up with the manager of Whites, at a rally and got him to try it. He lent me his machine and took mine back to the factory where it was finally sorted.
Coil choice depends on what you want to do. I would always suggest going smaller (8 or 6 inch) which will handle mineralisation better and get you nearer to rubbish items before target masking of want items occurs. I use a Bigfoot 22" x 3" on large dry sand beaches sacrificing depth for speed and ground coverage. Its light, similar to the stock coil. Largest I would use is a 12" Hotshot (not HotHEAD). This is another try before you buy option. The first I had was great but failed. The follow up just wasn't as good and I ended up getting my money refunded which was nice.
