Your GT may be falsing due to water and sand trapped under the coil cover, or because you have the sensitivity too high. Sounds like you are new to the machine, so have you tried hunting in Auto sensitivity to see how it works that way? If it's stable in Auto at that beach then obviously you are running sensitivity too high when in manual. You also must keep the coil fairly uniform in height while swinging. Also, try slowing down your sweep a bunch. Even if sensitivity is set right too fast of a swing will cause it to null and chatter. This is a slow machine. SLOW...Meaning probably 4 seconds per sweep, give or take a few seconds depending on how wide or short you sweep. In particular, at some beaches or land sites you've got to swing even slower because the minerals are so bad.
Again, great find! Well on you're way to paying for the GT. I guess by eyeing that ring you've at least paid for half of it and maybe even about 2/3rds.