99 S70 2.4 / nonturbo denso electronics, intermittant stall

[nonturbo]

After a P0420 code, began to stall like fuel being cut off momentarily enough to shut engine down... re- start no problem, changed to premium fuel seemed to help, but still trying to stall/shut down. 364k miles. I also noticed on start-up, idle goes up like normal then drops down below 150-250 rpm before settling in at normal idle range . Heard code refers to "Catalytic Efficiency Below Threshold (Bank 1)" Bank 1 ", last cel code was P1620. haven't seen any code since . Any ideas where to start before $$ on parts !! Thanks