whats going on with my potterton profile boiler?

most of my radiators upstairs fail to heat up when the boiler is on. All of the radiators downstairs are fine. When I turn all the rads off downstairs to boost the ones upstairs the boiler sends water up to the feeder tank in the loft which is consequently thrown outside via the overflow pipe. Ive drained the system a couple of times and put in sludge removal in case there was a blockage but to no avail. Any ideas?????????????