Macbook runs consistently hot sometimes, but consistently cool other times It doesn't seem to be a function of the programs I'm currently running, since I've run the same programs in both states, and it seems to vary beginning at startup: Most of the time when I start up it runs consistently at 180-210°F to the point of occasionally shutting down from heat, but sometimes when I start up it runs consistently at 140-160° regardless of programs being run.

Are there any startup items or background programs that could be causing this between different startups?

I have the fans constantly running at 6200 RPM using a fan controller to automatically set them at this speed and the fans themselves work just fine. So fans don't seem to be the problem.
It is not normal to have computer overheating to the point of shutting down.

You can look up in your Activity monitor to see who is consuming CPU.

Possible problems:

You cooling fan is not working at all or sporadic.

Your SMC (that controls the fan) needs to be rested.

Follow the instructions here based on your model to reset SMC.

You can also download the Intel Power app that will tell you what your CPU is doing.

The CPU is the single highest source of heat generation followed by GPU.

However, sometimes (rarely) the battery can do that due to the edit currents (internal) that will heat it up. Check you battery status and condition.