Glowworm 24CI Hot water problem?

MY system's central heating seems fine but it's having problems with the hot water.
The system fires up and seems normal, just as I start to get hot'ish water the flame dies down and goes out, three or four seconds later it fires up again, water has started to feel warm by this time, then just as it seems to get hot'ish again the flame goes and it keeps running in this way...
I have just blead my rads and pressure reads 1.1bar, just turned the heating on and it's gone to 2.5bar and started flashing, ive blead the rads abit more as only to stop the reading flashing, turned heating temp down too.
I've done that in the past after checking the rads for air and the system rectifyed itself last and held a good pressure even when hot.
Help, it's had a new heat exchanger about three to four years ago.