network hang using intel em nic

Hi,

I've got some new hardware and have been experiencing lockups using the em
driver. They seem to only happen on large downloads, smaller things like ssh
and web browsing work OK. The hardware is:

em0_at_pci0:0:25:0:        class=0x020000 card=0x309f17aa chip=0x153a8086 rev=0x04 hdr=0x00
    vendor     = 'Intel Corporation'
    device     = 'Ethernet Connection I217-LM'
    class      = network
    subclass   = ethernet
    bar   [10] = type Memory, range 32, base 0xf7c00000, size 131072, enabled
    bar   [14] = type Memory, range 32, base 0xf7c3d000, size 4096, enabled
    bar   [18] = type I/O Port, range 32, base 0xf080, size 32, enabled

I did have the vboxnet driver loaded, but tried unloading that and still saw
the issue. One thing I notice is some garbage at the top of the screen when the
hang happens, perhaps there's a conflict with the vesa video. (This system is
Haswell, using the vga/vesa driver.)

Any suggestions on how to debug?

Thanks,
Steve