Hot air fine when moving, but cool stationary?

[Fred Pitt]

Hi all.
Got a perplexing problem at moment, especially with winter coming.
I have a R75 Connie 2.5v6 petrol auto.
It's had a replacement stat a year ago, no leaks or drop in coolant level, but my mechanic fixed a leak from the short straight plastic pipe a few months ago.
I think he put some kind of rad fix solution in, which I wasn't happy about.
After that I had no hot air.
I asked him to flush out all the coolant and replace, and while at it flush out the matrix.
Since then coolant seems fine, no loss, and car runs fine, but hot air is fine when car is moving forward but as soon as you stop, (with engine running) the cabin air drops to just barely warm. Start moving forward and goes hot again???
When running the engine runs at around 81° which I know is a bit cool, but the hot air is fine when moving.
Air con was degassed and checked about six months ago.
Any ideas?
Thanks