Pump stopping in evenings - at wits' end!

We have an 8 year old Pulsacoil 2000 which has been largely trouble-free up until about a month ago when the pump jammed. A Gledhill-approved plumber came out and got it working again, but it had stopped again within a few hours. Plumber returned, drained the tank and replaced the pump. The old pump was completely seized up. For the next few days everything was well and we assumed the problem was resolved.

However, one evening a few days later we discovered we had no hot water; turned on the hot tap and listened for the pump but it obviously wasn't working. Instead there was an electrical buzzing sound. The next morning we had hot water and the buzzing noise had stopped - no noise coming from the unit when the pump wasn't running (ie when the hot tap was off). A few days later the same thing happened; stone cold water about 9pm, no pump and whirring sound. This time the pump started working about midnight.

We called out the plumber again; he said it would be caused by either the sensor or the pcb - recommended we try the sensor first as cheaper part. Again, for a few days all was well, then the same thing (pump off in evening, buzzing noise, working fine by morning). This week we changed the pcb - the plumber assured us this would be the cause as we'd eliminated all other potential issues.

But last night about 10pm we discovered the pump had stopped again. It was working by the early hours, but we're at a loss to know what to do now. We've spent a considerable amount on it and yet the problem persists - it's just an inconvenience at the moment I guess but we worry it's indicative of something more serious. We have good faith in the plumber - who has now changed the pump, the pcb and the sensor (the thermostat has also been changed). Could it be that the new pump is faulty, or might there be another cause?

Any advice you might be able to offer would be wonderful - we don't want to keep throwing good money after bad if there is a terminal problem with it.

Thanks in advance!

Edit: we live in a very hard water area (London) in case this is relevant

Hard water not relevant to pump problem.
I would say that the unit is sludged up.
Drain it down.
Remove pump and valves and see what you have.
When the pump was changed was it the complete pump with new body.
Or New pump head on old body?
Mike
MGC/H2O

Thanks for replying so quickly Mike.

When you say sludged up, what with? The plumber drained the tank when he changed the pump - would that have done the same thing (apologies for my ignorance!)? He replaced the entire pump in its casing.

Would that explain why it only seems to stop in the evenings? I realise it might just be a coincidence but it's a bit odd.

Sludge!
Bit of crap!
The pump has 2 valve at each side.
Those pipes could have sludge / hard bits in there!
Also on the lower part of the pump in the brass housing is a check valve!
You need to check the pump for voltage when working and not working.
I assume the correct pump has been fitted?
If there is a voltage at the pump and it DOES NOT WORK. Check by removing big screw.
Then go down the avenue I suggested.
I also presume that the unit has been wired and connected properly?
As for the PC 2000 as I know it needs.
A supply for the unit
An off peak for bottom heater
An on peak for top heater.
3 switches are the norm.
Along with a 30,60,120 minute timer switch for the boost.
The PC 2000 is the easiest unit to work on in my book.
Mike
MGC/H2O
Norfolk