see working jsFiddle: http://jsfiddle.net/ruslans/vFK82/
I have 3 fields: Net Price (ex. tax), tax amount and Total price (price ex. vat + tax amount). the NetPrice
Can't promise this will fix the problem, but sometimes enabling deferred updates can fix problems like this.
It's a good feature to be aware of, but be cautious about enabling it for an already working application - and also if you do have an underlying problem then you'll still want to fix that.
http://knockoutjs.com/documentation/deferred-updates.html