Ultra Monkey "Page not displayed"

Hi all,

We recently set up an Ultra Monkey load balancer with 2 real servers
and 95% of the time it seems to be working perfectly, but every now
and then our customers are getting "Page cannot be displayed" errors.
It happens at different stages on our websites and we can't seem to
reproduce the problem here. Out customers are very large Fortune 500
companies so we assume that their networking etc is top of the line,
and the fact that it is occurring with multiple customers we assume it
is our architecture. Our LB environment is as follows :

Ultra monkey box :
CentOS 4.4
ldirectord.cf :

# Global Directives
checktimeout=5
checkinterval=5
#fallback=127.0.0.1:80
autoreload=yes
#logfile="/var/log/ldirectord.log"
logfile="local0"
quiescent=no

# Virtual Server for HTTP
virtual=xxx.xxx.xxx.252:80
fallback=127.0.0.1:80
real=xxx.xxx.xxx.248:80 masq
real=xxx.xxx.xxx.249:80 masq
service=http
request="microsites/admin/UMPingServlet"
receive="Greetings"
scheduler=lc
persistent=2100
protocol=tcp
checktype=negotiate

# Virtual Service for HTTPS
virtual=xxx.xxx.xxx.252:443
fallback=127.0.0.1:443
real=xxx.xxx.xxx.248:443 masq
real=xxx.xxx.xxx.249:443 masq
service=https
request="microsites/admin/UMPingServlet"
receive="Greetings"
scheduler=lc
persistent=2100
protocol=tcp
checktype=negotiate

/etc/sysctl.conf :

# Controls IP packet forwarding
net.ipv4.ip_forward = 1
# Controls source route verification
net.ipv4.conf.default.rp_filter = 1
# Do not accept source routing
net.ipv4.conf.default.accept_source_route = 0
# Controls the System Request debugging functionality of the kernel
kernel.sysrq = 0
# Controls whether core dumps will append the PID to the core
filename.
# Useful for debugging multi-threaded applications.
kernel.core_uses_pid = 1


Real servers :
SuSE 10.1
Gateway set to Ultra Monkey eth0 address
Apache 2.0.x


Questions :

1: I assume that because we are using masq(NAT) that we don't need to
worry about the noarp problem with DR or TUN?
2: Is there any ip tuning that we should do on the Ultra Monkey box as
not only is it acting as the load balancer but it is also a router
too?
3: Has anybody else seen this intermittent "Page cannot be displayed"
error with UM?

Thank you very much for your help in advance,

Mark.