I have this happen on a daily basis; it started for me in late Feb after a Windows update.
As best I can determine it is a memory leak issue as stated in another reply. Unfortunately for me either the red lines or a pop-up saying I've run out of memory displays after about 20 minutes of use. That makes it rather difficult to enter research data. Hopefully yours isn't that frequent.
I typically shut down FTM (or it shuts down for me if I don't immediately shut down upon one of the warnings) and restart the program after a short time. Then I have another approx. 20 min window of slow-going work time.
Sad to say it's typical of a program that is poorly written with useless product support.