Radiator puking

Looking for ideas as to why my radiator pukes so much.
- Professionally rebuilt 318 grocery getter - about 500 miles
- new OER radiator
- new water pump
- new thermostat
- new 16 lb vented radiator cap
- new belts and hoses

Driving, you can watch the heat gauge climb in temp, then drop quickly - the cap is by-passing pressure causing the temp to drop, and creating a mess under the hood.

The coolant is red with rust, so I’m going to flush it all, replace the thermostat, use the proper mix and volume of coolant and try again. I have seen faulty thermostats before. In fact, I’m tempted just to leave it out. Where I live, we don’t drive in really cold weather that often.

Any thoughts on leaving the thermostat out?

Any other ideas what could be causing the puking and how to fix it?