TAX intermittently not displayed in admin product-edit screen

Hi, this keeps happening daily and it is starting to worry me.

In the admin side of the ZC site, on the product listing / creation page the automated tax calculation stops functioning.

picture attached, as a picture paints a thousand words.

I have checked a backup from this morning to one just done now via winmerge and there are no file changes. My hosts says there have been no changes to the server, no flagged problems etc or rollbacks. So i don't believe (hope) it is a server issue, database issue, hack / security issue.

Test orders placed via the front office show the tax. It just seems to be a problem in the back office (admin). Its becoming worrying as it might be a sign of something more troubling? Also, changing the 'Display Prices with Tax in Admin' to true/false/true/false etc has no affect either, at the time of tax display craziness.

After several hours, it then starts working again.

A browser shut-down, cache clearing or PC restart has no effect.

NB. the back office of my website is used heavily by 3 full time staff. Normally at least one admin user (me) is logged on for 3 to 5 hours at a time. Normally between the hours of 10:00-12:00 and 14:00-15:00 all three of us are logged on, amending stock or accessing orders etc etc heavily. In case this is an issue?

p.s. Got to go and pick the kids up from school and sort them out so might not be able to reply until tomorrow. Thanks in advance .

1.3.9H clean install not upgraded.

add-ons:
Category dressing
Image Manager 1.4
Edit Orders (latest version)
Easy Populate

The calculation is done by a small bit of javascript.
If your javascript isn't enabled or isn't executing properly, then you won't get the automated calculations.
It's also driven by the tax rates associated with the tax class selected along with the various tax calculation settings.

Thank you for the reply. Does seem to be a local machine issue. Our PC's are all going a little odd with the latest Windows 7 updates.