How to reduce a sites sudenly increasing bounce rates?

Hi there,

I recently updated a site that has been building in traffic with a new theme to clarify navigation and site performance.

The old theme was getting slower and messed up in usability but had a bounce rate of 5 to 8% with an average of 1.6 pages and 5 minutes page time. After the update the bounce rate went down to .25% pages up to 3.2 and same or a bit more on page time.

Two weeks later with no plugin changes but around the time of the latest google update the bounce rate went up to its now average of 86% which is killing the number of pages, time on site and page. I have been working on site performance which had already gone up from the old theme and testing plugin options but nothing is working.

Does anyone like @James Farmer have any ideas on how to solve this? The site is