When the problem happens, in the logs all you see is a very long delay (up to about 30 s) before the 'LockInOnSignal' messages appear after tuning. My guess is that the tuner driver is taking a long time to respond, but I don't know very much about what is going in TV Server in that situation:
[2018-12-19 16:11:32,749] [Log ] [25 ] [INFO ] - dvbubmiting tunerequest Channel:Euronews English HD subChannel:0
[2018-12-19 16:11:32,749] [Log ] [25 ] [INFO ] - dvb:using existing subchannel:0
[2018-12-19 16:11:32,764] [Log ] [25 ] [INFO ] - dvbubmit tunerequest size:1 new:0
[2018-12-19 16:11:32,780] [Log ] [25 ] [INFO ] - subch:0 OnBeforeTune
[2018-12-19 16:11:32,780] [Log ] [25 ] [INFO ] - dvbubmit tunerequest calling put_TuneRequest
[2018-12-19 16:11:32,795] [Log ] [25 ] [INFO ] - dvbubmit tunerequest done calling put_TuneRequest
[2018-12-19 16:11:32,795] [Log ] [25 ] [INFO ] - subch:0 OnAfterTune
[2018-12-19 16:11:32,795] [Log ] [25 ] [INFO ] - RunGraph
[2018-12-19 16:11:32,795] [Log ] [25 ] [INFO ] - subch:0 OnGraphStart
[2018-12-19 16:11:32,795] [Log ] [25 ] [INFO ] - dvb: RunGraph
[2018-12-19 16:12:02,029] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal waiting 20ms <--- this is 30 s after the previous log message
[2018-12-19 16:12:02,139] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal waiting 20ms
[2018-12-19 16:12:02,233] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal waiting 20ms
[2018-12-19 16:12:02,358] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal waiting 20ms
[2018-12-19 16:12:02,452] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal waiting 20ms
[2018-12-19 16:12:31,983] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal could not lock onto channel - no signal or bad signal
[2018-12-19 16:11:32,749] [Log ] [25 ] [INFO ] - dvbubmiting tunerequest Channel:Euronews English HD subChannel:0
[2018-12-19 16:11:32,749] [Log ] [25 ] [INFO ] - dvb:using existing subchannel:0
[2018-12-19 16:11:32,764] [Log ] [25 ] [INFO ] - dvbubmit tunerequest size:1 new:0
[2018-12-19 16:11:32,780] [Log ] [25 ] [INFO ] - subch:0 OnBeforeTune
[2018-12-19 16:11:32,780] [Log ] [25 ] [INFO ] - dvbubmit tunerequest calling put_TuneRequest
[2018-12-19 16:11:32,795] [Log ] [25 ] [INFO ] - dvbubmit tunerequest done calling put_TuneRequest
[2018-12-19 16:11:32,795] [Log ] [25 ] [INFO ] - subch:0 OnAfterTune
[2018-12-19 16:11:32,795] [Log ] [25 ] [INFO ] - RunGraph
[2018-12-19 16:11:32,795] [Log ] [25 ] [INFO ] - subch:0 OnGraphStart
[2018-12-19 16:11:32,795] [Log ] [25 ] [INFO ] - dvb: RunGraph
[2018-12-19 16:12:02,029] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal waiting 20ms <--- this is 30 s after the previous log message
[2018-12-19 16:12:02,139] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal waiting 20ms
[2018-12-19 16:12:02,233] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal waiting 20ms
[2018-12-19 16:12:02,358] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal waiting 20ms
[2018-12-19 16:12:02,452] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal waiting 20ms
[2018-12-19 16:12:31,983] [Log ] [25 ] [INFO ] - dvb: LockInOnSignal could not lock onto channel - no signal or bad signal