I spoke to VM yesterday as my internet connection had become unusable: it was dropping out every ~35 minutes. It appears that there is a firmware issue with the Hub3.0 which exhibits when in modem mode and connected to certain hardware at there end.
I got them to send me a SH2ac as it does not suffer from the same issue, and also appears to have noticeably better ping times.
here is a link to my BQM and my parents (Hub3.0 v SH2ac) both in modem mode.
Me: My Broadband Ping
Parents: My Broadband Ping
see this link to the VM forum for more details of the issue. (warning this thread is over 100 pages and counting, lots of people are suffering from this issue) Hub 3.0 port flapping
I hope this helps.