fxp(4) TCP checksum offloading

Hello list!

I got an Intel PRO/100 S NIC (Intel 82550 chip) which supports TCP 
checksum offloading. I am also able to load the microcode necessary in 
order to activate that feature:

fxp0: Microcode loaded, int_delay: 1000 usec  bundle_max: 6

Unfortunately, when enabled, network connections (at least those based 
on TCP) start timing out while ping remains working flawlessly. The 
system in question is running latest -CURRENT source as of today.

Also, after having put /sbin/ifconfig fxp0 link0 in 
/etc/start_if.<interface> (which by the way seems to be undocumented), 
the microcode gets loaded but then sshd refuses to start at boot time. I 
don't get any error message.

dmesg output:
fxp0: <Intel 82550 Pro/100 Ethernet> port 0xdc00-0xdc3f mem 
0xe3000000-0xe301ffff,0xe3020000-0xe3020fff irq 11 at device 10.0 on pci0
fxp0: Ethernet address 00:02:b3:4c:9c:32
miibus0: <MII bus> on fxp0

pciconf output:
fxp0_at_pci0:10:0: class=0x020000 card=0x00408086 chip=0x12298086 rev=0x0d 
hdr=0x00
     vendor   = 'Intel Corporation'
     device   = '82550/1/7/8/9 EtherExpress PRO/100(B) Ethernet Adapter'
     class    = network
     subclass = ethernet

Any ideas? (Is there a way to unload the microcode again besides 
rebooting the system?)