Just a few questions, what sensitivity setting are you currently using, as some areas will not play ball with setting 3 or 4? Also what detector are you using (if any), that could be causing some crosstalk between detector and pinpointer?
It's a bit hard to explain, but if you continually leave the TRX turned on, sometimes the pinpointer will just start ticking over on its own accord if set on higher sensitivity settings, just seems to feed off surrounding sources requiring it to be either re-tuned, or switch off/on again.
If you are getting black sands occuring in your crevices, the TRX may also be picking up on them vs true gold targets, just as some reactive ground/rocks may cause the TRX to start sounding off. As you mentioned you can retune the TRX to negate such effects, although it does reduce the sensitivity to the smallest of targets, which won't help when crevicing for small gold. Your only other option is to wind the sensitivity back till it is stable enough to pick up on the gold, whilst not picking up on the surrounding rocks.
You get a similar effect say if in my shed surrounded by electrical interference, where at first the TRX will be quiet on start up, but eventually continually tick over as it picks up on other sources. None of my other pinpointers really have such issues, or just to a lesser degree, in a way it is probably a trade off for having a much more sensitive pinpointer over the other brands. I really don't know why they bothered with a fourth sensitivity setting on the TRX, as it is unusable under most circumstances, even 3 sometimes.
Anyway, just a few things to consider, not sure whether they are the true cause of your issues, just my findings from owning a TRX.