Switching Themes, Current theme has issues, Best way to switch

My current site has about 250 pages and is on a WP theme that I purchased at themeforest.net. Probably 20-25 pages on that site have a different interior page design which was important to me but is no longer important to me. Unfortunately, I totally redesigned/destroyed the theme which has resulted in many major layout issues and browser incompatibility in the past and currently. I tried to upgrade to the most recent version of WP and it caused major formatting & functionality issues. Then I updated the theme and this also caused formatting and functionality issues. I have already done the SEO title tags and meta description on many pages of the site. I think I will need to a new WP theme and I have found a framework and theme that I like.

I am wondering whether it is recommended that I start from scratch and do a fresh wordpress install and manually transfer the content of each page over 1 by 1 as opposed to activating the new theme on the existing site which has already been problematic. Please advise. Thanks.