I really hope I haven't wasted your time... but here's the update - I pulled another brand new probook650 OOB and it connected fine. I used same switch / port / cable / everything. I then plugged in the trouble box - same platform / switch / port etc etc. - and it worked. And trust me that over the last several days I've reseated all cables in question several times...
again, sorry to have wasted your time. but I'm very thankful for all your help.
I wonder if I have a flappy port on the switch or an 'iffy' cable / connector??
anyway thank you so much :)