Magento - How to debug layout problems

I've just upgraded Magento from 1.4 to 1.5. The update went (almost) smoothly and admin appears to be all working etc.

The problem is with the theme. Something (a module or some bad code somewhere) is causing memory issues somewhere down the page (Error tried t开发者_Python百科o allocate memory etc). The top layout areas load ok. I know this because I get a fatal error on a deprecated method call and when I correct this error I see the problem

How can I track down the offending code/module?

Thanks

** EDIT ** The admin area is working just fine which is why I am assuming it is theme related

** EDIT ** I see the problem even with different themes

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If you can see the problem with the default theme, it's unlikely that the theme itself is causing the problem. This is backed up by the fact that you say that you have the same issue with multiple themes.

Assuming that you made no updates to the core code (right? right?), try replacing your entire core class folder with a clean copy from your version of Magento. Try doing the same with your default theme (if you have edits to the default theme, create a "new" theme with the default code and use that). Then, try again. If you still have problems, it's very likely that some of your custom code is causing the problem.

Hope that helps!

Thanks, Joe

EDIT: Make sure to disable caching, the Magento compiler, etc.