I have an old Servowarm boiler which has worked fine up till now but now the upstairs radiators are hopt but the downstairs are cold two of the rads downstairs are luke warm the other 2 are cold as well as the pipes into them but the pipes to the luke warm ones downstairs are red hot .The pump is making a noise like its working .l have just today put some sludge removal in one of the radiators downstairs. Any ideas thanks in antisapation. 
