occasionally can't paint in GIMP

It happens 50% of the time now - I can create a new document, but I cant paint.

Just trying to zero in on the problem as it is annoying in it's unpredictability.

Layers are unlocked, all Preferences/Folders are set to "writable".
I'm running the app from my local disc, root directory.
I notice the whole GIMPportable folder is set to read only, and when I untick the option, it usually changes back.

For the last 10mins, I have been going over these areas of the software to identify the problem, have re-installed latest version, and have been stuck.
I can do a background fill from the edit menu, but cannot paint, draw, select with "rectangular selection tool" , or fill with the paintbucket tool.

Just now I started it up and can paint again.

Any ideas on what could be causing this? Has been an on off problem the last week wile re-installing my system.
Anything appreciated.