Have you got a screenshot of the speedtest graph showing how it drops? And I assume that the "sync speed" as reported by your modem is a bit over 60Mbps?
This is just a guess, but it sounds to me like Zen could have applied a 40Mbps BRAS profile to your line.
By this, I guess you mean you've checked at BT Wholesale? It is possible Zen's own BRAS might be applying the wrong profile, e.g. a bug in their RADIUS servers.
If resyncing the line can make the problem go away sometimes, that would suggest Zen sometimes applying the wrong profile when the session starts, e.g. if they have a pool of RADIUS servers and one of them is misconfigured.