Blown Head Gasket?

[BillW]

Hi, I'm new to this Forum and to Supras. I recently purchased an 89 Supra, auto, normally aspirated, 160k miles and runs good. I think I have a head gasket leak, but I’m not sure and would like to be certain before I do the work to replace the gasket. Here’s the symptoms:
a. I’m adding a few ounces of coolant to the reservoir every few hundred miles and there is no visible coolant leakage on or around the engine and no noticeable smoke out the exhaust at least looking at the exhaust at idle and looking out the rear view on acceleration.
b. I’m adding a quart of oil every 400 miles. Again, no visible oil leakage on or around the engine and no smoke.
c. After driving down a highway for 10 minutes, the engine started to run rough like is was misfiring on a cylinder or two. I got off the highway and drove slower back home. Since then have taken it on a few short drives, and it has run fine, with no rough running of the engine
d. The oil looks clean.
e. I have not seen the temp gage go any higher than about 5/8 of full range at any time.
f. The person I bought it from has no knowledge of the head gasket being replaced.
The Toyota dealership told me “the head plugs are leaking” and would cost $1643. to fix. They didn’t mention the head gasket and I didn’t have confidence in their diagnosis, so I brought the car home. I then used a “block tester” kit from Napa to see if there were any combustion gasses getting into the coolant, but the test came back negative showing that there were no cylinder gasses getting into the coolant, at least when the car was only warm and idling.
I’m thinking maybe the head gasket only leaks when the engine is under load, like when going 70 down the highway.
Has anyone ever heard of a head gasket leaking only when the engine is hot and under load? If so, how would you test to see if the head gasket is the problem?
Would it be worthwhile to torque the head bolts and see if that fixes the problem?
Any thoughts are appreciated. Thanks.