Temperature Gauge and Thermostat
 

So I got the coolant hose issue on my Supra fixed at a mechanic, where they bypassed the engine block and hooked the two coolant hoses that feed to the top of it together, and it runs ok for now. That being said, I have had two incidents where I'm driving and then the temperature gauge goes up over halfway (one time it went almost into the red) and then back down to just below half. Is this normal, or is it something that should concern me? I looked elsewhere and read it might be time for a new thermostat. Should I do something about that?

Make sure your coolant level is between the high and low lines on the overflow tank. If your radiator cap is old replace it. Be sure to only check it or replace it when the engine is cold. As a matter of course, the level should be lower when it's cold and higher when it's warm in the overflow tank. This shows that the expansion and contraction of the liquid through the pressure cap is performing properly. The manual way to test a thermostat is to remove it and put it in a pan of water and bring it near to a boil. When the water is above 180°F the valve should be open. You'll have to drain the coolant in order to take it out though. Be careful of those 2 10 mm bolts that hold the housing cover on, they strip really easy. I use silicone as a gasket. The direction of the thermostat is important. The springy end needs to be pointing into the block where the hot water can contact the wax filled base that's round which expands on heating and opens the valve.

It could be air trapped in system... and yes it is something to be very concerned about.. I am in the process of replacing a BHG right now because I had a thermostat get stuck closed...