Sounds like the dizzy. Im having near identical problems to you. All cylinders (sp) work, all leads giving a spark and can rev the nuts off it fine when its not moving, but after a bit of a drive, it stalls at junctions and struggles to start up again. I put it down to damp at first and changed a couple of ignition components but the problems still persist.
Although i have a k10, the dizzy obviously only has one function so yours could be nackered aswell.
