Sounds A LOT like me to air in your cooling system. I had a similar problem, would have heat while driving then 10 mins later lose it. Turns out my T-stat got stuck closed a few months later, after overhauling the whole system the problem was gone.
If I were you, I would overhaul the entire cooling system (if you don't know when it was last replaced, then replace it). Make sure to bleed the system properly.
Start by running coolant temps via the onboard OBC, search on here if you need help on how to do so.
1990 BMW 325is - Stroker Build
2010 MINI Cooper S
1995 Saab 900 SE - JZW Stage 4