Revs drop when thermo's come on + crap idle help

[TheInterceptor]

Hello all! Here's a double header problem thread which may or may not be related to each other, so here it goes, both in one, sorry in advance for the long post!

So the story goes like so. When hot (of course) and idling in bumper to bumper traffic, the thermos will come on when they do and while doing so, it makes the car shudder, dropping the revs down to roughly 400rpm.

Previously in the past 9000 or so kms, i have cleaned the ISC, throttle body (while still on motor), checked for vac leaks with WD (produced no results), fitted new air filter, new thermostat, fitted new idler pulley, new plugs, new fuel filter, and a new fuel pump.
I did reset the ECU a few months ago when i bought the car using the instructions posted on this board. (unsure when the drops started, i have a great memory )

Havent needed to use AC much as yet but im sure that makes it drop a touch when engaged. Nothing else causes a drop.

I cant really think of anything i can pinpoint that of which would cause this to happen? Any ideas??

The car doesnt start perfectly every time either and the idle isn't perfect. Replacement of the idler pulley made it feel better, putting in BP 98 makes it start better and idler a LIL better, i suspect the servo i always went to has crap fuel as it pinged once apon starting and once every few times i punch the go pedal from idle.
I also noticed at idle the tensioner moves about with the rough idle, relevent?? I know its not supposed to budge! It feels like it misses every few revolutions. Its done this for a good while now. Car has 141xxxkms on it.

Any ideas? Think i might do a major service next and replace a whole bunch of things.
Cheers!