W60B base lost sync with handsets

[TrK]

We get 3 new W60B to replace our W56P. After installation first W60B, after some time (day or two) users starts complaining to no calls possible - handset (W56H with latest firmware) say "synchronizing, please try later". Dect base worked good - i can login to web interface, SIP registration are NOT lost, base is good, but connection with handsets is broken (and no sign of it anywhere, except "synchronizing" message).
After base restart everything was good again for day or two, then problem return.
Then i set local log level to 6, disable "ECO mode" on two handsets (we have 5 registered) and still waiting for this problem to export the log file. It was week ago.

Yesterday evening i replace second base to W60B and this morning problem returns with new base (where loging was not set to 6 and every handset uses "ECO mode").

What i shall do? Today i need to replace another one base and i`m afraid of this problem returns.