Oil Boiler gets too hot

[Mixmaker]

Hello everybody! I have little issue, and hope you will be able to help: I have very old oil boiler on conventional system. What happens is: When i turn central heating on, boiler is heating for about 1 hour, what happens at the end is pipes sound really loud(loud crackling sound) and overflow pipe flows hot water back in the tank on the loft, locking boiler from automatically turning on(i have to press reset button to run it again).I tried to lower pump speed on speed number 2, no luck. Also turned dial on boiler down to 3, still no luck. Any idea what can be wrong with it? Is there airlock in system somewhere? or something stuck in the pump? I have bleed all radiators and all radiators seems to be working ok.I have no idea where that noise is coming from before boiler gets locked, and why its overheating?

Any ideas will be appreciated.

Thank you very much for your help.

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[exedon2]

As you radiators are working normally and boiler is only cutting out on overheat (the noise is probably boiler actually boiling) first thing to look at would be boiler stat.
You could check that the end of stat is properly located in position . Some have a split pin keeping them in place others do not.