V8 goes cold on the motorway - fine in town

[DSLiverpool]

Guys the car heater works fine in town
In town water is at mid point
On motorway it can drop to stone cold
Not tried heater on motorway

Is this a classic stat issue or worse ?

It's a 2008 4.8 Cayenne

[DSLiverpool]

It's going in to be checked but a stat on these is manifolds off - I'll update

[DSLiverpool]

Code logged as stat stuck open - getting quote