im having a similiar problem with my mc21, just wondering is the sender found on the clutch side of the radiator?

my rad did have a heap of gunk (looks abit like rust too - will try and get some pics) in it though which i've flushed out with water, refilled with demineralised water... and will flush out that again until it clears up

could be possible that the sender is fouled, but the bike operates fine though, and doesnt seem to be overheating

*did have a quick look behind the gauge this morning, and noticed that the wires run in to the main loom, just not sure where it goes to though.