Unable to add new pages to Megamenu

Hi there,
Is there a restriction on number of items in a megamenu?
Each time I try to add a new page it returns this error:

503 Service Unavailable.
The server is temporarily busy, try again later!

I have spoken with the ISP who has checked scripts and increased time out limits and memory requirements at the code level and Lightspeed server. All to no avail.

Are there any known plugins that might cause this issue? (My site only runs a few)