As the above posts state it could well be dry joints or tripler failure. Check on the panel where the line output transformer is soldered in, if there are any poor joints they may arc and cause the set to trip off. It may be a good idea to re-solder them anyway as sometimes a dry joint is not obvious until you put an iron on it and the solder falls away.
To check the tripler disconnect the wire that goes from the LOPT to the tripler. Unsolder the wire carefully from the tripler end it is easy to break the wire off the LOPT bobbin which will ruin the transformer. Ensure the wire is away from other parts and switch on, if the set stays on you may need a new tripler.
However the focus control could be the fault and disconnecting the tripler will also remove the voltage to the control so if it does work with the tripler disconnected it may not be guilty . I say this because triplers usually fail and don't respond to being tapped. the focus control however might...
The focus units can suffer from bad connections, the tags and the earth pin are soldered onto a thick film track and can break away. Again, any arcing will cause the set to trip.
The tags and or earth pin can be re-soldered back onto the thick film with care but once done you have to be careful refitting the spade terminals as the tags are easy to break off again. The tags need to be supported when pushing the spade terminals back on.
When the set trips see if you can hear any clicking in the focus control .
Hope this helps, good luck with it!
Rich.