medway super water heater?

it seems to work fine for normal filling of sinks washing hair ,but if i try and run a bath after 10 minutes of constant runing it shuts off the pilot will not lite until 15 or 20 minutes later .i have changed the thermocouple and replaced the diaphram ,i know the pipes are not limed up as we do not live in a hard water area. the heater is about 6yrs old