Leth, as the machine warms up and 'settles' threshold may slightly alter. Components warm up etc, perfectly normal. Daytime temperatures etc may also play a part as does the type of battery pack used, eg 12v vs 16v. So yes when hunting the threshold may alter a bit, that is normal.
Failure to ground balance is either a faulty machine, operator error or ground so hot it won't allow you to balance the machine. Check your conductivity switch to make sure it is in ALL mode when ground balancing... The Tdi Sl has a very sensitive ground balance, listen for the high low audio response to assist you in getting it just right.
Get online and watch some of the Tdi SL videos to see how it is done, far better than the manual.
Let us know how you get on. Best of luck with your new machine Leth.. don't hesitate to to ask questions as it is a decent investment to make in a detector and it is important to enjoy your machine and not be frustrated by it..
Karelian