Strange behavior of USB PCIe add-in cards

Hello everyone,

I have two USB3 PCIe cards (different chipset vendors) that are both
exhibiting the same symptoms:
- They are detected and attached to the xhci driver.
- They sort of work (a usb stick seems to work for file IO)
- When an interactive device (e.g. USB sound card, mouse, or keyboard)
is connected, they seem to drop the overwhelming majority of input
events. For example, the mouse moves jerkily, at intervals of several
seconds. Most keypresses on a keyboard are missed, and occasionally a
"key up" event seems to be missed and the same key is typed
repeatedly. A USB sound card has stutters and hitches in the audio
output.

Does anybody recognize this behaviour or know of a solution? I have tried:
- A different PCIe slot
- Two different PCIe USB3 cards from different vendors (and different
chipset vendors - VIA and ASMedia). One of the cards uses the VIA
VL806, which some users on the FreeBSD forum have reported working...

I'm using CURRENT, but about 2 weeks behind...

Thanks,
Harris