Path MTU issues with Juniper SRX?

[polternet]

I've been using the TunnelBroker service for several years. It's migrated with me between homes and ISPs, which has been great. I am (and have been for some time) terminating the tunnel on a Juniper SRX210 running JunOS 12.1X46. About once a week, I stop being able to pass frames outbound from any of my internal LANs (3 of them). I get what looks like an MTU problem ("Packet too big" in pings, etc). Oddly, inbound-initiated traffic seems to be fine, so the behavior is somewhat asymmetric. If I deactivate the config stanza for tunnel interface in the firewall, leave it that way for about 5 minutes, then re-enable it, everything comes back. I'll attach the relevant config snippets below, but any hints on figuring out what's going on would be helpful.

Code Select Expand

[edit interfaces ip-0/0/0]
dan@mauer# show
unit 0 {
    description "HE IPv6 Tunnel";
    tunnel {
        source 99.X.Y.105;
        destination 216.66.22.2;
        path-mtu-discovery;
    }
    family inet6 {
        address 2001:470:X:Y::2/64;
    }
}

Thanks.
Dan