If you like to try some alternative, you could check out MP2. It won't interfere with MP1 and you get an idea, if it is a more a software or hardware problem.
I was accessing MediaPortal through Teamviewer which now has a default setting on it to lock the PC after you end the Teamviewer session. I didn't know this as MediaPortal was set to be Always on Top.
I've since changed this setting and/or stopped using Teamviewer and everything has been fine.
As for why I don't know, but I think this is the root cause.
Not sure if that is useful or not really, but it's fixed anyway.