Hi sem101,
Good to see you've fixed this already, awesome job!
I assume you have many plugins and pages active on your main blog :).
I recommend a lower value (96MB or 128MB) to prevent server memory swap overload when a bad script is present (a Sitemap generator can be considered a bad script, as it pulls out and rewrites whole posts database).
I hope this helps :).
Keep up the good work and have a wonderful day!
