Lennart,
If you are going to replace parts at random, you might be there for a while. Although replacing the electrolytic filter caps is probably OK, they are starting to age.
First off. The schematics you referenced is actually for 2 amps. One with tube PI and one with SS driver. I take it we are looking at the first schematic on that page that you listed.
Two of those 200 watt light bulbs wired in parallel should, but may not, do the job of dummy load, set the output impedance of amp to 8 ohms.. Solder some leads to them.
You will need an oscilloscope to properly test for oscillation. Most multimeters can't reliably measure anything above 500Hz.
Check the whole circuit board for signs of bad solder joints first. You can see bad ones with a magnifying glass: crystalline, powdery, discoloured, appearance of wire not actually glued to blob of solder, microscopic cracks between solder blob and circuit board, etc.
Really, you need test gear, like signal generator and oscilloscope to tackle a problem like this.
First task is to make sure the poweramp section is working OK. Disconnect R47 and apply 1 volt RMS sine, square or triangle wave signal to pin 2 of IC 8. Do you get a nice replica of that across the dummy load, or does it sound OK with a sacrificial speaker system. If yes then wait until it farts out and then test, or punch it really hard a few times to see if that causes bangs or pops in the sound (bang it with a rubber hammer if you have delicate hands), and so on and so forth.......
