Playback of MKV recordings drains VQ

When I record files on the remote (on lan) tv server running tvheadend in MKV format I have issues playing them back on Openelec through the LAN. LAN speeds are good and there is no bottleneck but at at some point it frequently loses all of the VQ and then dies.

Play back with .TS as the container does not seem to have this issue. Any ideas what might be happening here?