Ok, now I'm trying only with 5 nodes + WS and they don't seem to disconnect. Last night I was using 10 nodes and it was constantly disconnecting.
I'll try to keep these 6 computers busy for testing... mainly to check the best number of nodes to use (WS +2, 3, 4 or 5) to get a reasonable speedup.
EDIT: adding the 5th node causes disconnections. With 4 nodes it was working fine for more that 1hour.