ariston combi losing pressure?

i have a 6yr old combi
which if the heating and water are on will cut out or go to reset
fill boiler to 1.5 bar when stone cold as it warms up spits it out side then cools down to red under half a bar and starts messing around wooring but wont light till cold and the water is off
water stone cold this morning turned ch on water off
half hour later water was hot
pressure valve has been changed and dial british gas
say there must be a leak but no sign any where to warrant rapid pressure drop
when boiler starts needle jumps from 0.1bar to about 1.5bar
please help driving me mad
and british gas seem unable to trace

rob
January 2010