[Dominik]

blackjack,

you heard it all and from all sides and for a long time.

summary:
you started with heating up at speeds under 30 - 35 mph, in between it ran okay apart from the other day when ambient temperatur went up. now it is overheating over 30 -35.

for now I would buy bob's input about the slipping impeller or fan belt, or daniel's distributor pin theory if it also explains your intermittent problem.
also craig'd fuse / electrical problem seems alright (well, wrong for your car)

if the car is cooling properly in the garage at idle via the fans as you experience now, the amount of air moved through the radiator is sufficient. you may let it heat up to 230 F and than only switch the fans on, plus you may block 30% of the radiator's surface to check your (fans) reserve.

of course, there is no engine load at idle.

at speeds around 35 mph you shouldn't need any fans at all if the radiator is correctly shrouded.
worst would be that they switch on (which you can't hear), but still they should be cooling - if running in the correct directon!

you do not have a fan in front AND in the rear of the radiator, a puller and a pusher which may lead to (not blade stall) air stall. meaning no air is moved through the radiator. (that would not be intermittent)

the accurate bleeding is one thing, but how can you be sure that you keep all the air out once it is done properly? your sytem (ERA's) cannot be dimensioned that close to the edge (as was on the Pantera?)

my last Cobra only started to heat up once almost a gallon of water was missing (why that happened frequently is another point). so I had a good reserve.

the timing - again... it almost sounds too simple that noone can believe it may be that.
also check the distributor gear - but it would not be intermittent, or maybe sticking advance weights?

thinking of my buddy's Can-Am there is nothing fancy at the cooling system. a bleeder at the radiator and an overflow catch tank is pretty much it. not even a swirl pot. I hate to say: maybe because it's a Chevy??? ;-)

well I know, it's not funny anymore, wasn't probably after the second day.

what I would do now is:
bleed correctly
set the ignition advance to a permanent comfortable value, (35 degrees?) the starter will start, at least when it's cold. when it's hot wire in a way that you can turn the engine without ignition and switch ignition ON after it's turning (the Can-Am has a fixed 38 degrees, starts either hot or cold)

that should eliminate intermittent ign retard problems. monitor ignition with your timing light at various engine speeds - it should not change!, as well as fan speed to listen for rpm drop when the wires get hot (acustically or with timing light, second man needed)

or alternatively: after checking the timing, hit the street, aim for 35 mph plus where the fan shouldn't be needed anymore. you may disconnect them.

that should leave the slipping impeller or fan belt and the bleeding as a potential cause of trouble. but the real problem is your bad environment to work at the moment.

did I miss something here?

another day in paradise,

dominik, cape town