High idle at start then drops to normal 90q20v

[David Thoresen]

After punching out the cat and replacing the 02 sensor I have noticed 
my idle starts off rather high (1500rpm) then slowly settles down to 
800 rpm, til I rev up and then it repeats that process...  doesnt 
matter if the engine is cold or warm... I tried to adjust the idle 
screw but that didn't help any...

Could this be a problem with using low octane fuel? (still trying to 
use that up)  Or should I look elsewhere?

Thanks!
David
90q20v