After having solved finally a malfunction of my 600XL I would like to share with you some tips on how to T/S the machine.
If one day your 600/800 XL starts freezing after a certain period of time, giving you a black screen sometimes on power recycle then a normal boot or any other odd graphics effects (see pics) then you do have of course a problem but chances are this is related to memory management.
Before going through the painful process of swapping all main IC's (GTIA,ANTIC,POKEY,PIA) or even unsoldering them if your board is not one of the lucky versions with sockets then better is to start from what is simpler and probably more obvious.
-Check the PSU first of all and ensure that the output given is correct. Wait for the freeze to pop up before testing the voltage output. -Swap all rams as most probably one or more ram may be faulty. If on power up the built in memory test starts and you do see red squares then chances are one or more ram are bad and need replacement. -Swap the CPU as it has a lot of functions including of course memory addressing.
Before going through those step check the chip located in U13 in the 600XL (ref C060472-D). This TLL is a Memory Delay chip and has a very important role in the memory management. Any malfunction can result in all sorts of garbled patterns and freezes of your machine. This chip is also a common failure and changing it may solve your problem :-). Test it first before proceeding with the rest of the T/S! This has solved the problem for me.
Hope it will help users for any future 600/800 XL malfunction.