Carol: This may or not help, but one thing to keep an eye out for in cases like this is SNAT pool exhaustion if you're using snat on this virtual server. It'll show itself as random delays which affect overall performance (or worse in some cases).
If you're using a snat pool consider increasing the size to eliminate this as a potential issue. If you're using automap, definitely try using a pool with several addresses in it. 600MB is a non-trivial amount of traffic at L7 so I could see this as a potential issue if you're using SNAT here.
-Matt
Nimbostratus