1999 Audi A6 2.4SE auto doesn't start when hot

Car starts perfectly when cold, but when warm it just keeps turning over and never starts. Sometimes it does start but the rpm struggles and then it stalls when I put it into gear. If I leave it for 20mins or so for it to cool it starts ok.

I don't know if its related but a few weeks before the water temperature gauge on the dashboard stopped working, and just stays at the lowest value of 60 instead of rising to the normal 90 degrees.

Thanks