I don't think you are getting all of the air out - this situation can often occur after the system has been interfered with - a new radiator fitted for example. I'd try closing the worst radiator off, unscrewing the valves from the rad (NOT from the pipe work!!) and drain the rad. down. Then ,having a decent tray handy open the valves one at a time to let a good quantity of water out. If there is any air trapped, this should clear it. You would then reconnect the radiator and bleed it as normal.
John
December 2007
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