Just installing a CentOS 5.5 x64 PPTPD server.

Problem:
Remotely connected using a pptp client and could connect and login but not do an ls on a directory of files.

Solution
Reduce the MTU
edit /etc/sysconfig/network-scripts/ifcfg-eth0 (or whichever is your ethernet adaptor running pptpd) and add MTU= the value should be whatever the maximum size you can get away with.

# Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller
DEVICE=eth0
BOOTPROTO=none
HWADDR=20:cf:30:XX:XX:XX
ONBOOT=yes
DHCP_HOSTNAME=myhostname.mydomain.local
IPADDR=192.168.59.60
NETMASK=255.255.255.0
GATEWAY=192.168.59.254
TYPE=Ethernet
# added by james because ls ps vi top where hanging
# as soon as I tried to return more than a page of text
# manual command is:
# ip link set dev eth0 mtu 1200
MTU=1200