In addition to @mpa's super valid reference, use the search function a bit more, there are two or three other threads on Lantiq VDSL firmware versions. It's a regular point of discussion in here.
As for me, who is in the same situation as you (Telekom vectoring line): I never had luck with 126.96.36.199.1.7 -- which seemed to initially synchronize fine, but I was actually able to provoke to disconnect just by saturating it with a simple web-based speedtest. I had my best results with 188.8.131.52.1.7, with line uptimes somewhere around 7 to 10 days. But I still had constantly deteriorating line values before it broke down and reconnected. You will have to try out a few firmware versions and you may find one that works best for you.
But it seems that Telekom's lines (which invariably have a Broadcom on the other end) just don't play all that well with Lantiq modems. I was never able to get a truly stable line with a Lantiq modem. I eventually changed to an external broadcom modem (for me a dirt cheap Zyxel VMG1312-B30A) and never had any problems ever again. My line is currently synchronized at 116/43 mbit, rock stable without any deterioration, and going strong into its 6th week of uptime.
Edit: And I do mean rock stable:
(The blank area is where I had to inadvertedly unplug my router and lost a few hours of statistics. The modem carried on just fine, though.)