the radiator cap has to release for coolant to go into the reservoir (expansion tank). If it is not holding pressure then what you describe is exactly what happens.... Take the cap off and have it tested (or test yourself if you have the tool).
Haven't looked at the later model bikes radiator caps, but suspect it is a common design (unlike the older ones!) and you may be able to buy one rated for a higher pressure.
The coolant issue is a simple thing...your dealer has not found the root cause. I seriously doubt you have a blown head gasket.
As for your engine shutting off while riding....does this happen when engine temp goes extremely high? Just what are the conditions when it shuts off?
I used to be fast....now I just dream about it.
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'08 910R (Gina), '04 F4 SPR #275 Track Bike (Lola), YSR50 (The Rabbit), '71 CB100 (Lil' Red), '75 RD350C (Orange Bird) restoration project, '70 Suzuki F50 (Baby Blue)