Pilot Light Keeps Going out, Potterton Puma 80?

When the Heating has been on for a long time (4/5 Hrs), my pilot light goes out and the main burner also goes out. I then need to turn off my Boiler and put the pilot light back on again (as oer manual) and then switch the heating back on again.
Any ideas what causes this. The Boiler is very old (8/9 Yrs) and is probably on the way out but still works ok currently.
Boiler is a Potterton Puma 80.