Router that knocks off before we do

I posted a message a few minutes ago about a network problem and I am hoping someone will be able to help but I have another problem.

When our server was rebuilt we installed a new router.

Every single day the internet connection is cut. At 3.50pm exactly. The line has been tested, it's fine, the router has been replaced, four times, but still at 3.50 each day we are offline until the following morning...

All technical depts are totally baffled and we are getting increasingly frustrated. Any ideas?

Jen