Help, Vokera Linea 24, dilema?

Hi,

Please help as I have read all the postings on here and I am now not sure.

My boiler pressure is at 3.5, sometimes above and rarely drops. There is water dripping out of the pipe outside. When the shower is on, it runs hot then cold and so on, this used to occasionally run cold for a few seconds, however gotten worse and now only runs hot for 4 secs then cold for 4 secs etc. I have watched the flames in the boiler and they seem to cycle on and off. The temp rises and then the flames stop in which the temp falls and then the flames rise again.

After reading the postings, it would appear it may be 1 of 2 probs, Expansion vessel or ?? 'Plate heat exchanger' ?. I am confused, I have had someone out but they said it could be the heat exchanger and would replace it but there is a chance it might not work in which i would need to replace the boiler. Someone else has told me Expansion vessel. Is there anyway of really knowing. I just don't want to waste money on something that will not work. Many Thanks