boiler firing randomly

Hoping someone can help me solve an ongoing mystery of why my boiler sometimes fires up and sometimes doesn't... Worcester greenstar r1 boiler with an towerchron qe2 timer..it used to work fine, and respond reliably to the programmer but now only sometimes....why???!!! Having read various...

Problem: Circulating pump failed - new pump fitted - now the boiler seems to randomly fire up regardless of requests for heat. i.e. can turn controls to "Off" but boiler keeps firing. I have an elderly but usually reliable heating system. It has evolved over the years and now consists of: Ideal...