This has also happened to me too. as a matter of fact it happened this morning.
what I might suggest doing is to minimize system resources. shut a few programs down in addittion to lowering bucket size to around 8 or less.
also lower the raytrace size as well if it's more than 3. 

it will take much longer to render depending on size, but will be worth it in the end.

also if your image is larger than 1000 px in width I would suggest making it smaller.

good luck!