I also have a very similar problem with the poor picture and would like some advice from Simon at Now Tv. As this is my first month trialling it I would like to resolve this before I commit to using it regularly.
I have had this problem recently despite having adequate broadband speeds - I have (hopefully) solved it by buying a netgear wifi extender, plugging it in next to the TV and running an ethernet cable directly into the TV from it - and have had a smooth, sharp picture ever since.
I use the NOWtv app on an LG set.
No idea whether that's a permanent solution or I'm just lucky - but for people struggling it's worth a go.
If it IS a solution then NOW should be sending everyone a voucher to buy one if it's necessary to run their service properly.