Msg say SMTP Server timed out when sending email w/attachment; otherwise sends fine.

I have problems when sending an email with attachments (generally pdf). I get the error msg stating my smtp server connection has timed out. I can 'sometimes' reboot after saving the email, once restarted I can open the draft and it will go... sometimes not. I test the smtp server by immediately sending a 'test' email without any attachments and it will go right through. Open the saved draft and attempt to resend and it will fail. SMTP settings are correct and have not changed in several years and have not experienced any problems until this latest update to Thunderbird. I can use the same account at Go daddy's web mail application, send the same email with the identical attachment and it works correctly... so I don't think it is the 'attachment' or it's size. I can send 'test' emails to myself or my wife's account and everything works correctly during the session until I attempt to send an email with the attachment and it fails. Immediately send another 'test' email without attachment to the same account and it will go through. I am at a loss as to what to do next to correct this. I am contemplating an uninstall and re-install. No new software has been added to this computer and this problem just recently popped up. I'm stumped...