If it's a swap storm "vmstat 1" would soon show that. But if it were, the input during pause would be played back on resume. On the other hand, if it's really frozen solid for those 4 seconds, the kernel should moan in dmesg. My guess is hardware: open the machine and make sure everything (CPU, memory, PCI) is seated properly.

You can strike the nvidia module "off of" your list by booting to text-mode ("sudo telinit 3" or whatever) and seeing if it still pauses top.

Peter