Cannot ping host PC from VM

[xHoly-Divinity]

I've got 2 VMs (a ubuntu + winxp) running on my host win7 machine. The VMs are configured using a bridged network. They can ping each other, but not the host. I can even connect to the host from the VMs if I use the external IP (since i have port forwarding enabled on my router). BUT, I CANNOT ping using the LAN ip. Now, if I go on the host, and ping the VMs, initially it says that "Destination host is unreachable", but after a couple of seconds it starts getting a reply. And then I can ping from the VMs to the host (weird...). I can even cancel the ping on the host and I am still able to connect from the VMs. After several hours of inactivity however, the problem occurs again. Any ideas? Thanks