wizard-
Like everyone says, some form of directional antenna will solve the problem. I am using this at home to solve my BS hop problems. What you describe is defnitely basestation hop, your description fits mine to the tee. It is most annoying.
Assuming you have used panel antennas, I do have a mod for the one that iBurst sells that I devised myself, and which I have been using now for the last 5 months. I am not sure what effect it will have on any other panel antenna but it's worth a try.
With the modded panel antenna, I consistently get good transfer speeds and the link remains stable, typically a disconnection every 4-7 days, as opposed to every 30 mins. The panel is used indoors.
At any rate, if you can PM me your details I am willing to take a photo of my setup and send that to you in the hope that it can somehow help you.