i also had some reboot-on-packetbursts(especially downloads) problems with WPA that i didn't experience with WEP128 (i have 2 access points at home, one wpa and one wep)

i reported it to the bugzilla (search for WPA) but then a few days later i couldn't reproduce it any more.

also, with WPA the connection need a session of some sort, to authenticate (usign the passphrase) and get the current encryption key (which changes every hour or so, depending on the AP configuration) to encrypt the packets against. then the whole stream is encrypted.
with WEP, you supply that encryption key as 104 bits (=13 ascii or 26 hex chars) and that key is used to encrypt the packet data only. so i think it's quite normal for WEP to be faster. (but wep's not safe)