Outlook 2010: Intermittent problem sending Gmail messages

As I have a couple of Gmail accounts, I use Outlook 2010 to consolidate them together and use it for reading, sending and archiving all my emails.

Until recently I have been able to manage my emails without problem, but within the past few weeks I have run into problems sending emails using Outlook for my Gmail accounts. Messages are stored in my Outbook but will not send. This does not happen all of the time, only intermittently, and even if I press the send/receive button, or reboot Outlook 2010, the messages remain in the outbox.

The only way I have managed to send them is to reboot my PC and then restart Outlook - then the messages send automatically as they normally should do and the outbox is cleared. However after a while, and not on every Outlook session, the messages do not send from Outlook unless I reboot my PC again.

I have checked my email settings and as far as I can tell they are OK (as they normally work):
Incoming mail server: pop.gmail.com and port 995
Outgoing mail server: smtp.gmail.com and port 465
and outgoing server set to require authentication.

I get no error messages - the emails just remain in the outbox until I reboot the PC.

Can anybody advise how I can resolve this issue?
Many thanks for your help.

Not sure if related, but I am running Outlook 2007 on Windows 8 and have one Gmail and one Godaddy email account tied in (have had others in past but removed long ago), and I too am intermittently not able to get Outlook to send emails from my Outbox.

Normally, closing down Outlook and reopening it would solve the issue, but today I was completely stuck. I updated and rebooted my PC, and then after logging in and testing connection settings, it tests successfully, but the email never left the inbox.

I tried changing from outgoing port 587 to 465 but that resulted in it failing the test and then still not being able to send.

I changed back to 587 and restarted outlook and email is working again.

I just did some research that led me to telnet test on smtp.google.com 465 and it is failing to connect properly at all. If you use port 587 it works just fine.

By any chance are you using AT&T Uverse for an ISP?

No. I am on a UK-based ISP.

I have also tried deselecting the outgoing mail scan in my antivirus (Avast) in case this was causing the problem - it had no effect.