https browsing problems

My problem is that I cannot browse HTTPS pages using any of my 3 Ubuntu 10.4 (up to date software patched) machines.
They do browse HTTP pages fine, and worked OK with HTTPS for many years until recently - since when it's been driving me nuts.

2 of the machines dual boot windows (Xp on one and Win 7 on another) - and using windows I can browse HTTPS with no problem. (One wirelessly and one via ethernet.)

My internet is via a Billionaire ADSL router - which hasn't been changed in anyway, and I'm loathe to mess too much with this since the windows access is OK - so surely no problem with the router or my ISP you would think?

So something that Windows still does OK over the internet connection I have is suddenly tripping up Ubuntu.

More info on the backgound and what I've tried ............

The only system changes have been normal ubuntu updates. I've made no changes to the browsers config - except some attempts to resolve the issue.

I have tried 'telnet 443' to a google IP address and it works OK in terms of indicating that the port is not blocked on the way out.

I don't have ipblock on these systems or any firewall as such that I've ever touched or knowingly switched on.

There is no proxy server in use, nor are the browsers configured to use one.

ipv6 is disabled (in firefox about:config), SSL 3.0 is enabled etc (like I say no changes were made to the browsers - but I'll try anything)

I've cleared the caches, and I've completley de-instaled and reinstalled firefox on my EePC.

I'm generally using googles DNS servers (8.8.8.8, 8.8.4.4) across both Ubuntu and windows - and have tried changing the DNS to OpenDNS to no effect.

On firefox chrome and chromium it seems that the secure request is sent from the little message in the left hand bottom corner but then the browser just hangs until it times out - unable to find page message. Opera eventually returns the message "Connection closed by remote server". It looks as if the browsers don't understand what they are getting back. The sort of sites and trying to connect to - google mail logon, facebook logon, luanchpad logon.

In searching for previous issues of this nature I've found very few indications that an inability to browse secure sights is at all common, and none of the discussions I've found indicate any solution other than the sort of thing I've tried above over the last week or so (yes including switching everything on and off - quite a lot in fact).

I'm no expert - any advice gratefully received. I'm wondering if there are any system logs or special tools or other browsers that might give a hint?

Thanks