Hi,
seems like your clients are sitting behind a PAT/NAT device, and your are connecting to a PIX/IOS vpn client server, you can try IPSec/UDP(NAT-T):
pix would require 6.3.1
ios would require 12.2.13T
vpnclient v3.6 or later support it, make sure that NAT Transparency is supported.
thx
Afaq