Overheating/coolant loss on long trips

[dannydavi]

Hey guys let me break it down for you.

I have an 86.5 Supra n/a automatic. It currently has JDM engine in it as I replaced the old one after it seized up on me.

As of a couple months ago I've had to periodically replace coolant with no visual signs of loss. The more I drive it, the more I notice it mostly occurs when going over mountains up hills etc on long trips. I thought BHG already?!?! and looked for milky solution under oil cap, dipstick and during oil changes and everything was good there. I did a compression check and it was perfect (200-202 psi all cylinders on warm engine), also no white smoke out of tailpipe so I set my attention at other probable causes.

Before my next two hour trip I bypassed my heater core all together, no more hoses going in or out of the firewall. The reason I did this is because my heater would sometimes out of the middle of nowhere start blowing cold air... even though it was just blowing hot and I am still on the same trip yada yada... so I bypassed it just to make sure it wasn't the cause, and that next trip proved it wasn't, overheated again as soon as I hit a hill. I let it cool down then I took the thermostat out and put more water in and it got me the rest of the way.
Next week I had to come home and it overheated again. So bypassed heater, removed thermo, did common BHG checks, and replaced radiator cap to no avail, any suggestions would be greatly appreciated guys. Feel free to ask as many questions as possible.

NOTE: It was overheating around town at one point as well and that's why I replaced the radiator cap cause I heard air and saw water coming out of there. So no more around town over heating but still mountain/long trip overheating. Also the coolant loss might just be from the coolant overflow after it is running hot... cause I'm not sure where it is going.

EDIT: NOTE: Also think it might be important to know that my temp gauge would raise up slowly then plummet back down to normal several times in a row before it would get hot and stay hot.