As you have had recordings that wouldn't play, displaying the "No tuner available" message, and you have single programs being broken down into multiple files, I would say you have a tuner reliability problem. Either something has changed in the signal path to your tuner (antenna, cabling, the line of sight to transmission towers has been obscured) and the signal you are receiving is now weaker, or the tuner(s) is failing, or the power supply for the tuner(s) is failing.
The "No tuner available" message occurred because something went wrong in a recording process, and the tuner failed to tell MC it was free again. Rebooting the PC fixed that.
The multipart recordings could occur (I think - Yaobing can confirm or otherwise) if during a recording the signal is lost, so MC finishes the recording, and then the signal comes back, and as MC knows it should be recording that program, it starts recording again. (I know I can force multipart recordings to happen by rebooting the PC while a recording is in progress, and then on restart of MC the recording restarts.)
What sort of tuner(s) do you have?
What brand and model, specifically?
How old are they?
If they are external (non-PCIe) tuners, have you rebooted them?
Have you been having any issues with your electricity provider, such as Brown-Outs? (A dropped voltage on the supply line could cause a tuner to hiccup, while the PC manages to continue running.)