Intermittent hot water from Megaflow

Hi all,
New to the forum...hoping someone can help on something which has a CORGI engineer stumped...
I have a Megaflow 250 tank fitted (about 5 years old) with a Potterton Suprima 120. Now the boiler runs at the set time to heat the water and the tank is full of hot water.
However, when yuo open a hot water tap, it will flow hot water and then sconds later, cold water runs from it, then back to hot....on other occasion it will be piping hot water....and other times just plain cold. Turn it off and turn it on immediately and it pipes out hot water !
To my mind, whatever is in the mega flow tank should pour out. The tank is full of piping hot water yet on opening a single hot water tap, cold water is coming out ! But not always !!
I have checked the thermostat on the megaflow and that is the right temp.
Can anyone give any pointers on this one please ?
thanks talbir

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.