hi hoppy
I have a xterra 705 also and had a hell of a time with noise and false signals at the beach + black sand to check out some of the videos on mlotv.com i found some of there info useful there are good articles and videos on all minelabs there
I think alot of it comes down to trail and error read the manual lots, make sure you use the beach mode(the umbralla comes up on the screen when active) in coin/relic setting, it does help a bit i found that i got better results and less noise using prospecting mode on the really black sands and you can give the machine a bit of adjustment using ground balance offset function which can help to(you can use this also to try and minimize the affects of hot rocks) , just play around with the settings and if its not working out just do a factory reset and start over.
Hopefully some of this is helpful its only what i have experienced so far, i still have not got any gold with it yet, just coins, bullets, tiny bits of metal, junk etc
What coil are you running?
Cheers