I have the same problem. About two weeks ago I finally upgrade my server from v2.5.5 to v2.5.9. My issue with my PCH's locking up (black screen or frozen video playback) seem to have become a lot worse (more frequent) with v2.5.9.
Today I found that I have two PCH's which fail to run as NPVR client. I can select the NPVR app from the PCH menu, but after a few screen flickers, the screen goes blank for a while, then returns to the PCH menu. The PCH's were working (although poorly) yesterday. I tried rebooting everything, no dice. I'll get some log files together and NMTChecker results and post shortly. I'll also try the less-used PCH's and see what they do.
One quick question. It's been a while since I've done much in-depth trouble-shooting with NPVR. GBPVR use to start copies of the PVR service (forgot the exact name) for each client you had configured (i.e., the number of servers you specified to start). Does NPVR do the same? I have mine set to 5, but I just looked at the Processes tab of Task Manager and don't see 5 copies of any process in the list. ???
Server = i5-2400S 2.5GHz, 6GB RAM, 2TB & 2x 1.5TB SATA HDs ATI HD6450-512MB
Win-DOHS! 7 Pro SP1, NPVR v2.5.9
Tuners = Hauppauge WinTV PVR-2250-MC Dual Tuner
Clients = NMT x5 (PCH A-110 x4, A-100)