When huge buy ups happen, there is little to no lag, which in my eyes suits MtGox as this means they create definite panic buying. However, when there is a huge sell off there is instant lag upto minutes and in some instances an hour. I translate this as to Gox taking the high buy orders (therefore getting the best prices for themselves), and once trading resumes, panic selling takes hold and they buy back cheaper.
Now this assumes Gox converts or leaves their client trading profit in coin form, which I suspect they would. I highly suspected this for some time as do others but yet I see no outcry of this. People need to wake up and realise a large company like theres DO have great servers, and that lag is simply created by themselves to suit their trading habits.
I've heard crazier conspiracy theories. That said, you should take a long, hard look at Hanlon's Razor.