I can't imagine what would be different between extensions, if the ones that work "right" are also picking up line 8 when you lift the handset.
You could try using Copy, #399, to copy a working extension to a non-working extension.
So they lift the handset, it selects line 8. They have outside dial tone? When they press an auto-intercom button for another extension, it will put that line on hold, in preparation for transferring the "call" to the other extension. At this point, line 8 is just dial tone, and the phone company is waiting for someone to dial a phone number. While you are talking to the other extension, the dial timer at the phone company has expired, and now they are sending you the "please hang up and try again recording" or the howler, and finally the open loop disconnect signal, which will cause line 8 to drop.
But if they finish their intercom conversation before the phone company sends the open loop, when the caller hangs up, the "call" transfers, and it rings on the other extension.
That's why they should be manually pressing Intercom before pressing the auto-intercom button, or as Telecomboy says, press the auto-intercom button BEFORE lifting the handset.
Also, if their dial tone is derived from an IAD, most of them will provide the open-loop when an outside call disconnects, but NOT when the dial timer expires.