It's funny, éh? 

The vacuum gauge was the first alarm that something was amis.  I even connected an hand-held vacuum gauge to verify the dash-mounted gauge.

To be honest, I don't know if the valve guide is worn; I didn't pull the head (again).  The valves and springs were new and properly lapped upon assembly and I did pass a reamer through the guides just to make sure that they hadn't tightened up (manganese-bronze).  The rockers are roller-tip so I'm hoping that the guides didn't suffer too much.  I didn't notice much of anything on the rollers themselvs, save for the #6 intake rocker has a slight amount of scuffing on the roller.  It's not scored, just a bit of a blush on the otherwise shiny surface.  Sometimes I wish I had a camera in the garage, you'd see what I mean.

My suspicion is that the loose rocker post allowed a bit of flex in the thick-wall rocker shaft and that messed up the valve seating.  As rpm increased, the 'flex' became more consistant and it smoothed out.  Slow it down and the vibrational 'wave' re-appeared.

IF this was the cause, a lot of good came from it - new waterpump sitting in the spares box, electric fan mounted to the innerwing-side of the radiator, new head gasket, new intake, new carb, hoses, new lower back pain...

my favorite but rarely used tool - the vacuum gauge

rough idle, low vacuum reading, twitchy needle= intake valve leak. 

With loose rocker and /or valve,  was there a poorly seated valve or worn valve guide or both?

Not being a mechanic, I can't really add much other than to say I found your post interesting, and as I try to decipher it, I am left wondering why replacing 1 stud apparently fixed your issue.  Was it not previously allowing coolant to flow properly or a certain valve wasn't opening/closing properly due to the loose rocker pedestal (buildup of heat) or something like that John?  I think I better read it again.

Happy you got it though, whew !

I think I found the problem refered to in another thread, whereby my engine was running rough when idling (for example, at a traffic light), vacuum gauge reading 12-13"Hg and 'twitching', air-fuel meter reading leaner and leaner and temperature rising to astronomical heights.

All efforts to cool it were futile, supplement electric fan, 82°C (180°F)thermostat, new(er) MSC 2-core (supercool) radiator, new heater core, new silicone hoses, new head gasket/valves/springs/seals/etc., ignition timing 6°BTDC @ 1K, 29°BTDC @ 4K, rockers adjusted to 0.015", new intake, new HIF38, new water pump, new fan belt, new and different carb needles...

Stumped, I called the most knowledgeable people I know, conversed with several on this board and came up with a host of suggestions, but nonetheless, empty.  I'd tried the suggestions during the many sessions in the garage, and always came up empty.

So, in anticipation of pulling the cylinder head one more time, I figured, what the heck, bring the engine to TDC and re-torque it just for fun.  Well, that had no effect, and I figured, heck, while I'm staring at the rockers, I'll try re-torquing the pedestal studs.  Quick check in the manual, yep, 24lbs-ft.  So, still at TDC, loosen #2 approximately ½ turn, tighten slowly to 24lbs.  Move to #3, then #1, then #4.

I found that #3 wasn't holding 24 lbs-ft, so I removed the nut and washer to find the threads on the stud beginning to deform. 

So, I pulled a stud from the 'spare' engine, popped it in and torqued it up, fired up and Vroom!

Idle is now fine at 1K, vacuum gauge reads around 17-18 and stable, the air-fuel ratio still needs a bit of adjusting, but it's a lot better than before. 

The temperature is still rising slightly when stopped (at a traffic light), but it's also kinda warm outside right now and 92°C (197°F) isn't really too serious.