[j-nsp] Lost ARP/ARP limit on P-1GE-SX?

[billp]

Juniper M40, RE1, customer facing-interface is a P-1GE-SX trunked
into a switch (encap dot1q) in a colo environment.  Running 5.6R2.4.

We are having this problem where a single customer IP will become
unreachable from outside the network (but still reachable from
the router itself.)  I look up the ARP entry, it looks valid.
Clearing the ARP entry makes it reachable again from the 
outside world.  Then this process repeats with a few other
random IP's a few days later.

The customer is running everything behind a software load-balancer
(Linux Virtual Server), which means that all their IP's have the
same MAC address/ARP entry in our router.

We are currently at 5935 ARP entries, 5324 of which are this 
customer that is having problems.

I have looked to see if there are any known ARP table limitations
on the juniper web site, but have not been able to find anything
out.

Has anyone run into this before, or is this an obvious problem
having this many ARP entries on a single interface?

Yes, we have told the customer to buy a router-- but of course
they should never be expected to do that... :)

thanks
bill