I would say that this issue might not be easily noticeable to a common user ( at least before the JB update). As said by the OP, it is very much noticeable after the JB update.
I set up my friends HOX last night to connect to my wifi same issue it connects allows a little data then disconnects, entered settings in my previous post and all was fine. This problem is not specific to HTC, JB or ICS I have encountered this on previous android builds as well and had to manually configure the phones wifi to connect. In almost all cases it's software not hardware related.
Sent from...... The Milky Way
Just to make sure, are you telling that the WiFi disconnects? For me and OP here, our WiFi does not get disconnected at all. The device connects to WiFi and shows the WiFi Signal always (its connected in other words), but there is no data transfer stops randomly after sometime.
A new finding that i have is that if i leave it just like that, it connects back in sometime.