And guess what ..... now the speed has dropped back to around 3835kbps. I just cannot understand what the hell is going on.

New modem installed Monday 12th May - initially got a faster speed, then back to the same old speed as before.
Did several speed tests this morning at 9:10am - not exactly a peak time, and they all average about 3800kbps, with one exception of up to 6011kbps.
Any ideas on what is going on - other than the typically poor service we seem to get from VM.