Flow regulation - boiling the boiler...

I have a honeywell Y plan system.
The way it's been installed means that after leaving the boiler we first pass the expansion vessel and then travel to the airing cupboard on the first floor where the "birds nest" lives. Next is the heating pump, directly after that is a tee piece which allows a 15mm loop to the 22mm boiler return. After that is the 3 position valve feeding a 50 gallon tank and the 22mm heating loop.
The main problem I have is that occasionally the boiler boils - I can hear it doing so - it does cut out when this happens, but it's not good.
I have spent a great deal of time researching this problem on the internet and have read the DIY FAQ in depth - very useful.
As a first attempt to stop the problem - I changed the bypass loop referred to in the first paragraph for a 22mm loop and added a "differential pressure bypass valve" to that loop to ensure that there was no possibility that the system would be pumping against a closed head. Also fitted becuase becuase I don't think there is enough flow in the system - see below.
Every single RAD in the house has a TRV, (this hasn't affected the problem) - yes I know this isn't how they say to do it - but that's why I fitted a "differential pressure bypass valve". Often there are parts of the house that I don't want heated at all and this was the only sensible way to acheive this. Regardless of weather this is a good idea or not when the heating is on the boiler works as it should - and I can make it short cycle NO PROBLEM - which rules out any defects in the boiler itself. I should say that the thermostatic system is a CM-ZONE system from honeywell - but 3 rads have traditional TRV's on them and at least one of these is permenantly open (towel radiator). The run to this permenantly open towel rad is very long - this is why I properly balanced the system using the method outlined in http://www.diyfaq.org.uk/plumbing/rad-balance.html
So the conditions under which the boiler boils are - all rads closed except for the towel rad - and the boiler running at it's 50,000BTU setting as opposed to it's 30,000BTU setting - in which case it's fine. Or when were heating hot water only. I repeat that it has always done this under these conditions before I fitted TRV's everywhere and the Differntial Pressure bypass valve.
I did adjust the "differntial pressure bypass valve" to ensure that it opens at 0.1 of a bar - it's weakest setting and all parts of the house are fed well at this pressure - and also the thing is definately opening correctly when I was testing against a closed head.
The heating is on a 22mm feed and return, with 15mm feeds to the individual rads as I expect is the same in thousands of houses and seems to work fine. The hot water feed is 22mm from the Y valve. But, at the bottom of the tank someone has used a step down to 15mm to return spent water to the boiler return - this is only a 2ft run which connects to the main 22mm boiler return - amalgamating the bypass loop and heating return together with this spent hot water into one return to the boiler.
1. What was the thinking behind the original gate valve in the bypass loop, that was set 1/4 of a turn open?
Possible reasons: - Speculation:
1. The "differntial pressure bypass valve" is allowing too much *short* flow with not enough dissipation of accumulated energy though the system and the boiler thermostat isn't sensitive enough - for a quick rise in temp. 2. The 15mm step down in the return from the hot water tank is restricting flow too much and the boiler is boiling. I guess it was designed to ensure that the heating gets priority when both ports of the valve are open - is this a normall and sound idea?
Perhaps - thinking about it after a re-read - the problem is a combination of all 3 of the contributing elements I describe - low flow to the far reaches of the house, an insensitive thermostat and a short cycling return. Or maybe the DPBV is opening a little becuase the 15mm return is restricting the flow and therefore the extra pump energy must have to go somewhere - I mean this is hydraulics after all.
If point 2 is correct then the normall practice of having an open radiator will never work in my system and therefore I fundamental re-design is required.
Comments and solutions very welcome.
Thanks in advance.
Lastly (if you've got this far - well done) this has to be sorted out becuase I want to scrap the existing Apollo Fanfare 30/50 si - old noisy and not powerfull enough - and install a new cast iron boiler in the slightly damp cellar. Slightly damp means it's workshop with tools which are fine but you could not keep paper down there. Has anyone any experience of the pros and cons of doing this. I should say all the pipe work at the moment runs into the cellar and the from there to the rest of the house and therefore it will be simplicity itself if only I can sort out the flue. I need according to diy faq an 88,0000 BTU boiler.
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