How can we mend our Baxi 105e combi boiler???

We have a baby on the way and pray every morning that we can have a hot shower!! We cannot use the heating as after about 1/2 an hour it cuts out and an LED starts flashing below the 50 degree symbol. We have looked in the manual and it tells us there is a flue/pump problem but have had 2 different plumbers out to look at it and none of them have any answers! We were advised to change the thermistors so did and the problem just won't go away. Another problem is that if we use the shower and turn another tap on while it's running both will run cold and never heat up again. The only way we are getting by is when the LED is flashing turning the boiler off at the plug and leaving it for 10-15 mins and trying again - which doesn't always work! Can anyone help please???