Interestingly, if I use the circuit tester function, there is a
circuit present between the rad pipe I was using as ground and the
case. Could they both be at the same potential maybe?


They could both be at earth potential. E.g. your radiator is bonded to
earth, your computer case is connected via the earth from the PSU.
Maybe it's *you* who isn't at earth potential. Could you be touching
anything else when you get the shocks? E.g. your table lamp is live. You are
leaning against it, but are well insulated so no current flows. You touch
your (earthed) computer case.... you fly across the room. I'm assuming that
this isn't what is actually happening, since you don't mention flying across
the room, but I think the concept has potential (sorry)