Vaillant Combi Boiler - Getting Hot Water Intermittently

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I have a 3 year old Vaillant combi boiler. Not sure of the model, but it is the type with the lcd digital temperature display on the front, along with an analogue clock setting doobrie.

This problem started today, as a bath full of hot water was ran last night. No changes have been made.

If I run the hot tap, the water heats up slowly, never getting truly hot, then cools down again. This happens continuously. The pressure gauge is at 1.3 bar.

The CH appears to be working fine, and I tried switching it off to see if that would help, but it didn't.

Any ideas would be much appreciated.
 
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FIrst, with the heating off, and pipes (especially the far left, fat one) cool, turn a hot tap and see if that pipe gets warm while the tap is on.

Also, leavng the tap on, how high does the temperature go? Does it go up and down with the tap water temp?
 
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The far left pipe does get very hot with the tap on, and the temp gauge tops off at about 81 deg c. and stays around there.

However, with the temp on the gauge ramping up (from about 40 before the tap is turned on), the tap water goes from cold to warm to tepid to warm.

There is a pipe with a stop tap that connects the two fat pipes going into the boiler. Looks like its there to bypass the boiler. But what do *I* know? I think it should be turned fully clockwise, but I've tried it at full lock both ways and there is no difference
 

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