Infinity normally requires MTU to be set to 1492 and not 1500.
Is your modem unlocked and what are the figures there for potential and actual data rates?
I also hope you mean Megabits (Mb) not millibits (mb) ...
And one thought - check the settings on the switch - have you inadvertently a port (or more) to run at 10Mbps? On my Linksys/Cisco is is easy to set ports to run at a restricted rate.
What happens if you plug your PC directly into the UTM10?
M H C
taurus excreta cerebrum vincit