Ford V-10 Techs Needed, Diagnos My Erratic Coolant Temp

There's no reason 4500 RPM will cause overheating. It's designed to run that fast. I do it, and much faster, for extended pulls up long grades.

Since you recently had cooling system work done, I'd be suspicious of an air pocket. If that's the case, it probably purged itself. If it does it again, I agree with swapping the T-stat.