Whoah, so many great responses. Thanks all! I’ll respond to each individually below –
Barkydog – Are there any specific colors/textures I should look for when I check the oil dipstick?
Tester: I just ran your test, but totally forgot to leave the radiator cap off. Will report back tomorrow morning when I re-do it.
GeorgeSanJose Yes, I was about to get on the highway when I noticed my temp. gauge running up and down like mad. Brought it into my shop (NOT the shop mentioned in my first post), and they said my radiator was busted. It’s been about 5-6 months since the radiator was replaced, and I haven’t had any issues with the temperature since.
Re: over-filled cooling system & my first post – When they reset my reservoir, it was still WELL above the “FULL” line. Please advise.
I did look under the radiator cap just now – no gunk detected.
VDCdriver – Not sure if she meant to point out an associative rather than a causative link. It’s a small, full-service shop attached to a gas station.
Cavell – Not really sure, as I hadn’t worried about my radiator until I noticed my temp. gauge on the fritz. There were no overheating issues prior to that incident ~5-6 months ago. The head gaskets were replaced by the used-car dealer – who used the machine shop across the road; on hindsight, I should’ve yelped them! – after I got a third-party inspection at Subaru dealership, who noticed something that may or may not have indicated a gasket issue.
Honda Blackbird – I hope the above replies (especially to Cavell’s and GeorgeSanJose’s) help further contextualize the issue w/ my radiator. I am also thinking that it was either a) mediocre mechanic workmanship, b) mediocre machinist workmanship, or c) a combination of both.
Again, I will try Tester’s test once more tomorrow morning – this time, I won’t forget to leave the cap off and check for bubbles! Thanks all for your help. 'Til tomorrow.