Ok. Thanks for your help but it looks like its garage time! I was hoping to repair it myself but it looks like i've hit a brick wall. I'm pretty sure i've cleaned what i can and there are no leaks as far as I can tell.
Why would this only occur after going over 40mph? Its a strange one.
Find a (very) quiet flatish road, and drive along at 30 - 40 ish. Keeping your foot absolutely fixed on the throttle; if needs be, hold your other foot underneat the throttle to stabilise it. If you feel the car pulsing, you may have a sluggish lambda sensor.
If you aren't confident to juggle about with your feet like this when driving, please ignore the suggestion.
A sluggish lambda sensor would explain that you have no problems from cold - closed loop operation using the lambda sensor doesn't happen until the exhaust gas warms up. It would also explain the engine not adapting quickly enough to returning to idle.
This is just a possibility - make sure it is the lambda sensor before fitting one.