[Plugin: Contact Form 7] Error on IE

Contact Form & works for a few days and then I hear from someone that they tried to submit and got the red error box. Today it works in Firefox but not IE. Last week it was IE working but not Firefox. Have used the Plugin for 3 years and only in the last few months have we had this issue. And yes I have swapped the “old” contact form code and “new” contact form 7 short code in and out repeatedly. It will fix and then stop a few days later. Any ideas?

http://wordpress.org/extend/plugins/contact-form-7/