I can't really say I ever noticed a reproducable and definite change in behaviour of the 716-series between the official 5.3.3.5 Tele2 firmware and later versions. Not enough to validate a real hope that your problems could be solved by a firmware upgrade at least.
I don't see that this would be the main problem : a modem which suffers constant reboots and loss of sync will mostly turn out to be a defective modem, unless it would turn out your linespecs have recently changed in a drastic manner (due to external causes like damaged lines, work-in-progress or God knows whatever can happen in the outside world), it will probably be solved by swapping the modem.
That swap will undoubtably leave you with a 706 instead of your current 716, which isn't *that* big of a problem really. Owners of an original 716 are officially excluded form changing one Iota of their device-settings (hence your precious 716 with known Admin code, I suppose), but 706 users can alter their modem config (on firewall/routing/portforwarding level) by using the somewhat limited Tele2/Tele2 login codes.
And if that, for some reason, whould not be enough to satisfy your needs, well, erm, how do I say this :
"what gets on the internet, stays on the internet"
"seek and thou shall find"
"good things come to those who .... (muffled voice) ask"
By the way : as English obviously isn't your native language, why not use that instead ?
So indeed, it's a small world. 
