The experts should answer to you soon but the little i know it can be upstream congestion because VM confirmed at their forums in the past for similar problem i had
Upstream use QAM 16 and is only in use one channel at the moment so seems most of the cases is due to upstream
Look my graph how UPSTREAM congestion changed after reset SH and changed upstream channel from 358000 mhz to 45.000 mhz
Downstream channels and hoping upstream sometime later when will add more will be for everyone
EDIT
Of course if your area has too many users can cause and over subscribe congestion as well due to heavy downloads