The idle should begin at about 1100-1200 momentarily after starting, then quickly sellte at about 750.

"for about two minutes" is too long for it so stay low, and 550 is low under any circumstance.

Suggest you recheck the throttle lingage adjustment and throttle positioning switch settings, for openers. Maybe even check engine timing.

Then concentrate on the idle control valve. Sounds like it is sticking.

While the RPM is at that 550, try rapping on it with the handle of a screw driver. If this "unsticks" it, remove and clean the output end by blowing some carb cleaner through it.