Third Party access to gmail

I am having a problem of third party access to my g-mail account---at least, this is what Google security reports to me, with a recommendation to change passwords etc.

I do the necessary. This is confirmed by Google , and all should be secure. However the reported access take place again, and again I go through securing my account again and again. However I waste too much time in front of my computer already and I would like thi sequence to stop!

To be clear, I have not seen any evidence of third party activity on my account, but access is reported typically in London, which I know could not possibly be me! For a time I thought hat this might be some microglitch, BUT, recently I began receiving emails from the GOP (Republican Party) , not addressed to me, which I have ignored and mostly deleted. These emails, are mainly about fund-raising, but indicate some system fault somewhere. SO--somebody accesses my g mail account (Apparently and as reported by Google), and I am getting some body else's email...

Naturally I am concerned! What can I do, beside abandoning the email account?

Anybody out there seen this problem? Could Mr Google do a bit more than send automated responses--Is anybody alive in that organisation?

Cheers

Barry McKinnon

I have a MacBook running 10.6.8, and an iPhone 4s with current software