caddy life 1.9BLS DSG 2009 over cooling issue?

[CaddyIssues]

i have been chasing an over cooling issue with my caddy for a while now, i have put a new genuine vw o ring and thermostat. the van seems to over cool itself when i put the heater heat to max and also fans to either 3 or 4 speed setting, the engine will then cool down like 3 notches on the temp gauge. i know these engines produce very little waste heat but surely vw designed it to able to hold its operating temp when the heating is set to full. otherwise why have the faster speed settings. i have owned loads of diesels and always they have heating up fine to operating temp despite running heating max (these vehicles did not have an aux heater either)

i know the caddys post 2010 with dsg gearboxs had a secondary thermostat. this pre face lift doesnt have this so that is also ruled out. i dont know too much about egr coolers but could this maybe be the issue? id be very greatful for some advice from someone who may know? it will reach temp fine on low fan settings and once left that way for 5 mins it will hold heat fine WHILE DRIVING, when i idle it then losess the 3 or so notches of heat over like 10 mins


sorry to ramble so much but i just wanted all the details to be in my initial post to stop replies about things i already have tried (that are all written here)

thank you!

[STP]

My CAYD does the same thing, even when at full temp it will cool noticeably if left idling even with the heating on. They are just so efficient that heat is only produced when working against a load, and when idling it can drop quite a bit after a few minutes.