If you raise the buffer to 1024, do you still get the same dropouts?
Are you using a template made in Logic 7 originally?
Do you hear audio drop outs in the same part of your song every time you play it?
What audio interface are you using?
How much space do you have on your mac's HD?
Also, if you have it turned on, try turning off your Airport. This has been reported as an issue that affects Logic's performance.
If there's still no joy, try repairing permissions:
Open Disk Utility (Applications/Utilities)
Select your Mac's HD
Click on repair permissions.
When finished restart your mac.
Let me know how you get on.