I wonder if you could borrow a same type of box from somebody to see if it still happens with a different one?

Other thoughts...
I suppose you checked all wires and connections already, even where it plugs into the wall.

Did you try the turn off everything-for about a minute- - modem, computer - and reboot?
Or try the little reset button on the modem box-
{a little inset hole area - push it with a paper clip or similar sized thing}
and then reboot computer too?

Did you buy the box or rent it? If bought how long ago?
maybe in the warranty time frame??

If you check all the above and still gets worse I'd agree that it is the modem or possibly the wiring or where it plugs in.
Have they sent anyone out to check the wires?
If a rental modem they should send you a replacement- maybe even if you bought it.

If you don't get any help go up the chain of command next time you call - or email the head guy.