so i ended up contacting the gentleman on the german ebay and bought the "matrix" T5200 from him, to use as a parts-donor for my own T5200 with the parity error 2 message. While doing this, as i wrote earlier, i found one morning when i tested my machine that the parity error was not there anymore (it counted nicely to 8MB of ram totally). So now i have this new "matrix" machine at home, and should my own machine behave, then this one do not really need to be a parts donor.
But this crazy "blockiness" got me intrigued so i started out with some experimentation. First of all, i made sure that all ROMs are actually in place and in contact with their respective sockets. Then i pulled the "casette" from the back (with controller + vga card) from my working machine and inserted it into the "matrix" machine. The result: Still playing scenes from "The Matrix" even with my "working" VGA card. Vice versa, my working machine still works fine with the "matrix" grahpics board inside.
I guess this leaves me with the advice given here earlier by a fellow member, namely to recap the PSU (i should probably try to measure it first using my voltmeter). If that will not work, then i assume its either dry-joints (could it be that??), something fishy with the plasma screen itself (recap?) or the ROM thats there right now (??)
Behind these "blocks", i actually do see some text output from the boot process (the usual bios battery message and the option to press F1). Not chrystal clear, but its there alright.
Inside the bios, i can see that this machine differs from my other, working, one in that it has (i believe) the award bios instead of the phoenix one (can tell from comparing images uploaded in this thread earlier). Another odd thing is that the matrix-machine has got a 500MB hard-drive inside it (which i guess either means that the bios is old enough to not have the restricting checks in it, or that the bios is modded somehow).
What would you gut feeling be? Recap first or put new ROM in it?