05 4.4i Heating Issue

[CalgaryX5]

Alright i put this out to tender a few days before it heads to the dealership.

When i request heat, say 32 deg (max in this example) and I am not in Auto mode - simply entering the temp desired and setting the fan speed myself,
I get NO heat at all at speeds above about 90km/hr ... 50mph ish...

It isn't until I slow down in the lower speeds say below 60km/hr that I start getting heat again..

Through all vents when I am on the highway I am getting cool external air. even if 32 deg is demanded.. again not in auto mode.

Outside temp during this time is -10 deg celcius on average....

coolant levels all good / blower motors replaced last summer.

Has anyone seen this before ??


JMW