I have had the same problems for the last few days and managed to get an engineer booked pretty quickly.
1st up I called on Wednesday night as it had died completely. Spoke to someone in the Indian support who was OK and got me back online fairly quickly. I mentioned about the dropouts I had been experiencing and he asked me to monitor it.
Yesterday it was dropping out every 5 minutes so I called again and spoke to someone in Wales who checked the modem and noticed the SNR was causing a problem (I think mentioning I used to work for them and had checked all the basics helped!) and arranged a visit for tomorrow afternoon.
I was offered an afternoon today or morning tomorrow as well. So if everything goes according to plan and he turns up and fixes it then it'll be a big
to Ntl from me.