GRE Issue

So I was wondering if people have had the same issue with GRE that I am currently having. We have GRE tunnels out to a 3rd party vendor that filter web traffic and I am getting this weird issue that whenever something goes over the tunnel quick, like I ping the dest router over the tunnel, 2 pings get through and the rest drop untill it stops for 3-4 seconds. I've messed with MTU sizes, and I get the same thing whenever I try to do an extended ping to figure out a good size. I'm starting to wonder if there is something weird on the ISP's side blocking this. I have another tunnel using IPSEC but that isn't having any issue at all. It's just the GRE tunnels. For refrence the ISP is comcast

Type escape sequence to abort.
Sending 10, 100-byte ICMP Echos to 10.10.10.10, timeout is 2 seconds:
Packet sent with a source address of 10.10.10.11
!!........
Success rate is 20 percent (2/10), round-trip min/avg/max = 24/28/32 ms