Midi Dropout

A while ago I recorded a couple Sonar X3 midi tracks (in different projects) using a Nocturn keyboard input and Cakewalk TTS soft synth. Everything working fine until recently when I found that both midi tracks intermittently dropout and in a very inconsistent way - you can hear some notes, play it again and you can't hear the same notes; or you can hear other notes which previously you couldn't hear. All very weird. I've not changed anything since I've recorded them. The track settings haven't been changed; the audio and midi settings in Preferences look ok. I've never had this problem before and I simply can't work out what's happened to make the sound intermittently disappear.
 
I changed my audio interface to a Scarlett 2i2 several months ago and it seems to work fine. In any case both projects played back perfectly through it until a couple of weeks ago. I also upgraded to Windows 10 last year, but again I've had no problems with these tracks until very recently.
 
I'm baffled and any ideas would be welcome.