I have a 99 a4 with the 1.8t
It's temp stay norm in town but as soon as I get on the highway the temp starts
Climbing and as soon as I slow down it cools down to norm
What could this be
Please help

 

Could possibly be the water pump and/or check you radiator for leaks when its pressurized.

 

If it was a water pump
How can I test it

 

Can you see any evidence of leakage, near timing belt cover, under engine, on the ground?
Another place to check is your hoses. A failing radiator hose will collapse when the pump is pumping hard. No leak, just collapse like sucking on straw too hard. Squeeze your radiator hoses, any lack of true firmness means it might collapse under high volumne. Replace the hoses first before going deep into any work. They should be cheapish.

 

No evedence of leaks and I'm upper radator hose is
Kinda soft when cold like I can pinch it till both sides are touching with eas also my g12 level is at min