potterton puma ?

my potterton puma which is about 8yrs old has developed two faults, when you run the shower upstairs for more than about five minutes it makes the boiler go to lockout you then have to reset it and then it will fire and run again for about another five minutes the water gets really hot before it cuts out, and the other problem is that there is little or no central heating the boiler fires for about 30secs and then cuts out if you feel the ch outlet pipe it is hot but the water dosent seem to go anywhere. we have had an engineer to flush the system but he seems to think that there is a blockage in the pipe upstairs and the floorboards need to come up . the heating did work until about a month ago and now we can only get some of the radiators working some of the time. the bolier sounds like it is just sort of humming all the time when the heating is switched on .please can you help we are not sure if to go ahead with the expense of taking up the floorboards .