I believe this is likely a network issue but I'm having difficulty tracking it down. I had a very similar problem a while back that seemed to be from having too many NFS shares. I'd reduced the number to 4 and it cleared up.
I had another similar problem later that I tracked down to a failing network switch. Replacing it got things back to normal.
I reduced the number of NFS shares to 2 when the problem came back but that didn't help.
What happens is the disk light goes solid for several minutes, during which iotop sometimes shows some activity from virtuoso-t and jbd2, but not huge amounts. Dolphin becomes unresponsive for the longest period, but any other program may also stop responding. The mouse cursor still moves and terminal sessions remain active. Top shows my computer is loafing along.
I'm running Jessie with a KDE desktop. I update daily.
I've looked at iperf stats and they show I'm getting the expected speed to my server (three switches away). My Internet speed is also what I'm paying for.
Any ideas on how to track down the problem?