2001 V70T Auto

I've noticed recently that my car's temperature gauge doesn't climb
quickly up to "straight up" and stay there.
This winter (London, fairly mild:  2 to 10 deg C (not freezing)) it
seems to be quite sluggish to get up there. Is that normal? Or should
I replace the thermostat?

Also, and maybe this is related: Once up to temp, when sat at traffic
lights the idle sometimes drops to the point where the headlights and
dashboard lights dim a bit, then the idel kicks back up to 800-900 rpm
and stays there.
Its like a tired driver fighting to stay awake - it almost 'nodding
off', but catches itself just in time, and then stays alert for a
little while longer ('til the next stop light).

Does any of this sound familiar?

I had the throttle body module (or was it gearbox?) reprogrammed a
while ago, and that cured all its previous idling woes for about a
year.

TIA

Dz