Harware specs could be an issue. I've got twin copies of SME 6.0b2 on two different machines. One is a quad processor (4 x PIII 500 Xeons) with gobs of RAM, the other is an older PII 200 with 96mb of RAM. I have had many issues with the install on the PII that never occur on the quad processor machine.
That being said, it's unlikely that the machine itself would cause a script error, but it is possible that the machine is unable to process your mail fast enough and therefore your timeouts occur. I think this is what Ray is getting at. Perhaps it's also got something to do with your connection? A slow connection trying to receive a large email may be a factor?
I did find, on both machines, that having SpamAssassin installed before Amavis caused problems - Amavis was working, but SpamAssassin was not (although it worked before installing Amavis.) Curiously enough, checking the headers of incoming mail shows that SpamAssassin has started working again in concert with Amavis.
When I run "top" from the console on the quad machine, it usually only displays .3 or .4 % usage on one of the processors, rarely higher. That being said, it's not that easy necessarily to watch the load change as your server is receiving mail. Perhaps you could test by sending mail to your machine from a Hotmail account or the like and then keep your eye on "top" for changes in the usage. That might give you a clue as to whether or not your hardware is being overburdend when handling mail. It might be fine when it's idle.