How to inspect DSL router?

How to inspect DSL router?

There is a DSL connection and a DSL router (Speedport W 701V). After some
days or weeks (I didn't record the events) the router makes some problems.
Internet connection is ... broken but not completely disconnected. E.g.
requesting a website the page either gets "displayed" very slowly or does
not beeing "displayed" at all. A retry sometimes results in more success.
So the connection isn't completely gone, but very slow and nearly
unusable.
It is almost the same when I request the router's config web interface.
A ping to google and to the router itself, both result in massive package
loss.
That's why I think, it is more a router problem. And in fact: If I restart
the router, everything works fine again, for weeks until the same problems
occur again. This is going for years now.
I personally don't know why this is happening. Looks like a software
problem (on some level) of the router, because it is not permanent and a
reboot solves the problem temporarily.
But to do something I need more information. Unfortunately the routers web
interface is limited and does not show anything suspicious. That's why I
ask:
How can I get more information about what's going on there?
Here are some approaches that came to my mind:
I may capture some traffic with ethercap / wireshark and analyse it, but
that seems to be complex and time-consuming.
What's with ICMP? I never used much more than a ping. Aren't there tools
with which I can inspect the routers internals?
Any hint's on cause / root of the problems are welcome, too.
P.S. The router's firmware is up-to-date.