I'm not sure whether this plays a role or not, but your Marathons are significantly overinflated according to Brompton's recommendations:
Schwalbe Marathon
Front: 60 to 75 PSI, 85 PSI max.
Rear: 70 to 85 PSI, 85 PSI max.
Here is the link
http://www.brompton.co.uk/page.asp?p=3069