Hot Coolant light

So at 189,000 miles my 2010 prius had a head gasket leak resulting in misfires. I used copious amounts of Bar's Leaks Head Gasket repair poured directly into the engine coolant reservoir. To my surprise it worked, I stopped losing oil and the misfires ceased.
I have since drained the Coolant/Bar's Leaks mixture and put another 14,000 miles on the prius.

However whenever I'm on a longer highway trip going up a steep hill north of 65mph, my Engine Coolant Temperature light comes on and blinks until I'm over the hill and let off the accelerator. It also occasionally burps up some coolant through the cap on the coolant reservoir. My thinking is the Bar's Leaks Head Gasket repair solution has gunked up something inside the engine that is causing this.

I'm looking for any tips on what to replace to fix this problem. Do I need a new radiator? Water pump? Any help greatly appreciated.

 

Problem solved. A new radiator fixed the issue. Swapped it with a buddy and it took us about 3 hours. I've put a thousand miles on it since and it hasn't belched any coolant or flashed hot coolant light once.