Baxi Genesis 80 combi hot water not so hot

My hot water gets hot then cold then hot then cold and so on. To get a bath full of hot water we have to run the tap very slowly. Having a shower is out of the question at the moment. (shower comes from hot and cold taps)
I have taken out the secondary heat exchanger and cleaned that. It was not very dirty.
What would be the order of looking for next to cure this problem?
I am guessing, flow switch then perhaps diverter valve?
Is there a way to test these before I go to the expense of replacing them?
All suggestions gratefully received. :)
Steve...
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It's not so easy to clean the secondary (i.e. DHW) side of a plate exchanger. The deposits are magnetite which is doesn't dissolve readily in nomal descaling acids, and the tight geometry is hard to clean mechanically. My mate's brewery uses Paraflow plate heat exchangers which can be dismantled for cleaning.
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In message

I can't picture the genesis atm, it'll come to me later
does it have a modureg? Is it working properly
--
geoff

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I have ordered some Furnox DS-3.
Which would be best practice?
Take the heat exchanger out and clean that or leave the heat exchanger in place and treat the whole system for around 3 hours or so?
I have also ordered a MagnaClean Central Heating Cleaner.
Steve..........
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Anyone?
Steve......
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