sudden problem with mail app

After using the mail app effortlessly for months for my gmail account, I'm all of a sudden having lots of problems. The main problem is that messages won't send. Sometimes I'm asked to put in my password (which I never did before), sometimes it just tells me it can't connect and won't send. Sometimes I don't have any problems. In short, it's being very temperamental and most of the time won't send mail (but does usually receive mail).

I've double checked all details several times and I've also uninstalled and reinstalled my account twice. I googled the issue and saw that a few others had reported similar problems, but couldn't find any solution.

Does anyone have similar experiences or some advice?