Oil Fired Radiators

Hi, I have an oil fired Stanley Super 80 which runs the radiators in the house. It's a pretty old system but the boiler is regularly serviced. The problem is rads that don't heat properly and in particular a towel rail radiator in the bathroom where there is no heat at all, not even the pipework connecting from the floor gets warm. It used to semi heat up, just the lower half but nothing at all now. I have bled all rads apart from one in a bedroom which has the bleed nut at the back and I just cannot open. Some rads heat very well, some only partially but most let out no air/water when bled. Last night I turned heating on full blast, after 90 mins turned off all rads apart from the towel rail, it sounded like the whole system was going to blow up, really loud air/water noises from the system. No heat at all rushed into the bathroom (as I had hoped), I got worried it might do real damage so opened the next worst rad (the one with the bleed nut at the back that I can't open), water rushed into this one but, as usual, did not fully heat, bottom part hot, top still only warm.
Does anyone know if I were to repeat the above and just let the system make all the noises would it eventually sort itself out or is it just going to blow up?!
Thanks