Greenstar CDi combi boiler overheating?

Hi, Greenstar CDi combi boiler 3 years old. Top floor radiators cold so bled, this dropped water pressure to zero so recharged back to 1.2 bar and system working fine. Then started to overheat intermittently when turning on tap for hot water (when heating is running). Led display rises rapidly form 60,70,80 etc until it tops out at u9 (which means 109 as only 2 digit display).

System then cools and is OK, the next day the same thing will happen. Any ideas?