You may try this and below is a link as well.
Check your logging on the pix and look for messages such as this:
4/7/2006 08:40 10.4.1.70 Warning Apr 07 2006 08:40:41: %PIX-4-419001: Dropping TCP packet from outside:68.15.199.241/80 to inside:66.70.69.249/55646, reason: MSS exceeded, MSS 1260, data 1452
I ran into something similiar with remote desktop it would attempt to connect and go to an hour glass and stop.
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a00804c8b9f.shtml