Potterton Performa 24 DHW issues?

My Potterton Performa 24 has developed a DHW fault, & I need help resolving it please.
In November, it stopped producing hot water and heating, & had a new divertor valve diaphragm and a new PGB fitted. All was fine for a while.
Recently however, when using hot water, the flow is luke-warm at best. When turning on the tap, the boiler ignites for 15 seconds, then switches off for 11. Then on for 15 seconds, off for 11. And so-on. The water supply temp goes from (very) briefly hot, to arctic, and back and forth. Having scoured the internet for a remedy, I've read several possible causes, as follows: Faulty thermistor. Divertor valve actuator. Divertor valve microswitch. Circuitboard or loom. Heat exchanger. DHW sensor.
While the hot tap is on, the l/hand 22mm pipe becomes hot, which tells me the divertor valve is sticking, but why is the boiler going on & off at those regular intervals?.
Assuming that the divertor is the cause, do I need to change the entire divertor assembly or just the actuator?
As you no-doubt know by now, I'm pretty bad at fault diagnosis so any & all help would be greatly appreciated, many thanks.