My 1655 did the same thing and then (fairly recently) the onan had it's last run around the yard! She punched a hole in the cylinder wall... Time for a harbor freight Predator re-power! As to why it was increasing in RPM the longer it ran, I was leaning towards a govene\or issue but never got to investigate it fully! Good Luck with yours... I wish they all had Kohlers in them!