Having the same spiking problem, gets quite annoying after a while.
Already cleanded it out, it was full of dog hairs and dust (which probably sticks to your socks and gets caught in the pedals). Didn't help at all.
I was wondering though, is logitech's warranty any good, I'm not sure if it's still in warrantly, but I'd like it replaced.
Possibly get it replaced for a G27 with an extra payment.
That's why I'm wondering, even if i'm not sure about warranty, how good is Logitech's customer support actually?