Under XNA, the garbage collector on PC was rudimentary compared to the generational garbage collector on Xbox 360. I had the same problems when working on a game in 2008.
I think you got it backwards, the GC was rudimentary on the Xbox.
Ugh yeah, you're right - I had to look this up again, it's been that long. Makes sense as we always ran it on the machine as running it on PC would run fine.