In all probability, yes. There can be other causes (in the network) that manifest themselves when there is heavy load - these are sometimes indicated on a Thinkbroadband.com/ping graph by showing dropped packets that might be down to upstream line noise.
It would help if you posted a daytime/peaktime TRACERT
www.bbc.co.uk. Also set the TBB graph going and in any case post your power levels, as you never know what can be deduced.
But dropping to the 40-60 meg range seems to me to be a peak time likelihood.