As you mention other hardware/drivers work better, it seems to be a problem with urtwn(4), probably only in connection with slhci(4). Can you test your urtwn(4) with another machine (with a "proper" OHCI/UHCI/EHCI) to prove the latter?
Apologies - I postponed this when I thought I sent it.I've tested that very urtwn and other urtwn on other machines (amd64, evbarm) with occasional issues (sometimes I get messages about firmware not loading), but usually they work fine.
Before Nick fixed it, umass(4) with slhci(4) had a very similar problem that small transfers (block sizes) worked, while large ones broke it after ~1 sec. slhci(4) is a strange beast, I also have PR kern/51664 open for a USB 10/100 ethernet adapter (axe(4) at slhci(4))... Nick is probably your first address here, he also has ISA hardware. :) http://www.flxd.de/isa-ws-sl811/
Thanks! John