Have had to reflash CM 11 on my Nook HD+ several times So I have Cyanogenmod 11 on my Nook HD+ and after just a month or so it starts performing awfully! 
To the point I can't use it and have to reflash CM 11. I've had to do it several times in the last 5 or 6 months.

It will reboot itself randomly. It's also so laggy sometimes I can't even unlock it because when I swipe to unlock it, it either won't register the touch or the animation will seem to be so slow it turns the screen off before it gets to the lock symbol and I won't be able to give it anymore input.

I guess my question is: have I done something wrong with the flash or is this my HD+ dying?

On a side note after I flashed CM 11 for the first time if I let the battery die, I won't be able to turn it back on for days until it decides I can use it. I avoid this but just not letting it die.
Also the screen will sometimes turn to "snow"...? I'm not sure how else to describe it, it happens quickly but it looks like noise/snow on an old TV, and it usually happens on the lock screen.
CM is a very interesting mod. I have had the same issues. One of the options you can try is when it gets laggy is to change the theme from the Hex to the Holo (android default) and it can help with the lag and lack of response. 

That is what I did and I have not had issues. 

Source: My OnePlus One and my past