1) Keep in mind that this is an unsupported configuration - RMS is not designed, intended or licensed to run across a WAN.
2) You're timing out because it takes too long to transfer the entire item list across your remote connection. The item list can be rather slow to load even on a LAN as your item count increases. I have a client where the NIC on one register kept downgrading itself to a 10Mbps Half Duplex connection, and Store Ops exhibited this same behavior - Item and Customer Lists took several minutes to load. 10Mbps is quite a bit faster than most remote connections, and HQ Manager loads more item data than SO Manager...
3) The only possible solution to this is to increase the upstream bandwidth at your HQ location, but you're still going to be in an unsupported configuration and there is never going to be any guarantee that you can connect HQ Manager remotely. Reducing the number of items might get you connected, but isn't really a real soultion, is it? If you select the option "Always use Find for Items", it MIGHT help, but you're probably out of luck.
I have a couple of clients who've tried this, some successfully and some not.
Good Luck!