[c4koh]

The engine got to 260 for not more than 10 minutes, and I can't be sure it was that hot when driving it. Coolant was always fine (water never boiled off).

In any case....

Suspect #1 from the threads above re: injectors. I just went and tested the resistance across all of them, and...

(I read it should be 12.1ohm +/- 0.1ohm at 68F. Today was 90F (the engine probably a bit warmer too, as I tested about an hour after a short drive to get coolant to 210), and none checked out at 12.1ohm, the "good" average seemed to be 12.9ohm. My notes on this (I think they're from Marc Haibeck) notes that 13.5 around 30 minutes after warm is a good average, so I presume my 12.9 is about right given the time/temp.

For reference:
PRIMARY INJECTORS
#1 12.8 ohm
#2 13.0 ohm
#3 5.8 ohm
#4 12.9 ohm
#5 13.1 ohm
#6 13.0 ohm
#7 12.8 ohm
#8 12.9 ohm

SECONDARY INJECTORS:
#1 6.3 ohm
#2 0.7 ohm
#3 11.4 ohm
#4 3.4 ohm
#5 6.3 ohm
#6 12.8 ohm
#7 8.7 ohm
#8 5.9 ohm

As can be seen, Primary #3 looks bad, and pretty much all except #6 of the Secondaries look poor!

So certainly will replace these, and while under the plenum fix the vacuum leak. I hope that with no evidence (so far) of white smoke, oil in coolant, and coolant/froth in oil, that hopefully (fingers tightly crossed) that the rumbling is just a mis-fire from the injectors not fully functioning...

This being said, for my own interest and knowledge (will try and look up the answer in the meantime):

A "bad" injector (concluding it's bad from the off-par resistance) still functions, just badly, right? And perhaps in my Secondary #2 (resistance at 0.7 ohms) it's just shot?
When they're "shot" - do they let in any fuel (as basically drips rather than nice injected spray) or do they simply clog up and not allow anything.

Still unsure why the car drives fine and powerfully until it's at least warm, and seemingly with no problems (and very, very fast) with POWER KEY on.

Thanks all so far for the help. Will report back once I've replaced the injectors.