Problems Pinging Win2003 Servers

I have four Windows 2003 Server (standard) boxes at my ISP (they are
colocated). The four machines route through a Microsoft switch which
goes to the ISP feed.

Each box can ping out, but I'm having trouble pinging each other, or
pinging them from outside.

I can ping the gateway... no problem. I can't ping the machines from
the gateway.

If I am in a box, then I can ping it from outside... but as soon as I
leave the machine, it progressively looses the ability to ping. Within
a minute or two, it can't be reached.

Now here is the part I'm not too familiar with (I looked over the
support guys shoulder as he was confused...), my IPs using ARP (?)
don't have MAC addresses. When he does a ping of my machine (and when
it does respond), some of the pings report a MAC address, but some
don't.

Here we thought that the network card was crapping out (on all four
machines) or some-kind of load balancing was running (with one card)
which caused it to go in and out. I verified that load balancing was
not configured.

I've tried the following:

1. I made sure no firewall is running. I went to Advanced from the NIC
configuration menu and I went via RRA.

2. I verified that no load balancing was configured for the card.

3. Verified with my ISP that my IP, subnet mask, gateway and DNS
servers were all configured properly.

4. I disabled everything I could find about sleeping. The NIC was
allowed to go to sleep, I disabled that. I removed the screen saver.

5. Checked the BIOS and the network drivers - all are up to par.

6. Changed the configuration of the NIC card from AutoDetect to
100mbps half duplex.

7. Disabled any Plug-n-Play settings in the BIOS of the motherboards.

I have a similar situation with a .NET server running at home, nobody
can ping it and occasionally my IE doesn't download all the graphics
on a web page as if the network connection is coming and going.

I'm going to try to swap the switch - it is the quickest and cheapest
thing to try ($35). All the hardware, cabling, etc... is new, I don't
understand it.

Am I missing something?

Thanks.