Wow, thanks for all info.
I think as long as the potential difference is the same between live-null, and live-ground, I can conclude the ground is properly ground being the same potential to null. Right?
So I guess the most probable scenario is that radiator and the pipes were kind of live by unknown reason. But keep in mind the building is from the 60's 10 floors tall, 12 flats per floor and all radiator are interconnected. So who knows what happened over the years to become live.
The tingle was not static.. it would not go away. Anyway for many years I had this cable from the radiator to computer case. Since the tingle was obvious while resting my feet on that radiator and touching any metal part of PC (and having a metal keyboard it was hard to avoid). So this solution worked OK.
Now I can't do any testing because the problem is gone or isolated from me. I know for sure that few years back I have measured the voltage and read 220V AC from case to radiator, and that the radiator would light up a pen-tester-thingy, barely. I don't know what sort of amps were running there but no breakers tripped. (and about the breakers.. we have ceramic fusible fuse cartridges, and not automatic breakers)
But here there are unfortunately many improvisations of all sorts that kind of worked, but still shoddy and unsafe. And sometimes true incompetence. (I remember a big one made by the electrical company: they routed to homes two phases and no neutral.. that was fun but nothing was damaged in my house but my neighbor's TV did not survive.)
Safety is not tight as in other places like UK (120V, polarized plugs, child finger safety, fuse inside plug.. etc), but I guess makes people a bit more aware as there are not many incidents regarding electricity.
Other odd practices. When you don't have an outlet nearby, but you have the junction box at hand, just make an extension from there and twist the wires bare hand ONLY one at a time. - this practice was common for cable-internet providers to power a corded drill to fix various switch boxes on the hallway where are no outlets.
You all know it's not recommended to touch the live wire, but if you are insulated from ground it's quite safe actually. (Don't try it though)
I do have decent knowledge about electricity, but nothing fancy in electronics, yet I am a mechanical engineer, and transformers, motors, generator, batteries, etc were thoroughly studied. General wiring of a building was not.