: does anyody know WHY it happens ?

: Is it because of the interaction between my body & the seat ?

: or is there some other reason ?

Last year I was doing neuroscience research for an Anatomy & Physiology professor and I asked her the same question. She said it wasn't due to lack of blood flow to the feet, but rather a lack of blood flow to the nerves (sciatic nerves) that terminate in the feet. She said it was due to my body weight not being evenly suported over the whole of my butt, instead, it was concentrated on two small areas, one on each cheek. Unfortunaly, the sciatic nerves are beneath these areas, and so, have too much pressure applied to them, and your feet go numb as a result.

: should I blame the seat ? or should I blame something else ?

It is the seat. Perhaps you can adjust the position of the seat slightly, so that it supports the backs of your thighs a little more? I found that this relieved the problem for me, though I'm using a different seat than you.

: how can I tell if the same problem occur with a different seat before I
: install it ?

Not sure that you can, maybe someone else has found a good seat and can recommend one.

Hope this helps.

Paul.