heyhey, i have the mx300 (same optical engine) and a funcpad as well.
I personally haven't had problems, but a friend of mine had the mx300 with the func as well. He had a similiar problem with the mouse curser going retarded. He i think now uses his mouse via PS2 because that's the only way it'll function properly. His USB port was the problem i believe. We switched mice, and the problem repeated, but i had no trouble, and we eventually deduced it down to the USB port.
We tested the pad to (i.e. we switched) and did all sorts of combinations, didn't use the pad-his mouse-my comp. etcetc. And the only one thing that was constantly broke was his computer. So yea, it probally isn't the pad, but i'd recommend using the rough side of the surface just so there is less to 'scan' optically. *shrugs*
As for the left click, try installing the newest drivers? if you have those, then uninstall the drivers? That's a real funky problem, not sure what to do there. But yea, test the mice on another computer somewhere else (i.e. give a friend the mice for a day, see if they notice anything) and if it's still buggery, call logitech, time to RMA.