Intermittent power loss, Ford Transit Diesel?

My 1999 Transit 2.5 DI is suffering an intermitent power loss. It seems to be caused by air getting into the fuel lines, as bleeding the injectors of air cures it (temporarily), although not fully -it always feels lumpy at low revs and slow to pick up from idle, but otherwise fairly good at higher revs... until it happens again-

The power loss usually occurs when the throttle is lifted off, and then suddenly there's no power there, you can stand on the throttle and doesn't pick up at all, or very slowly.

You can often bring it out of this condition by changing down a gear to suddenly raise the revs and it sometimes then starts to come back to life... of course, this is not always practical (need a bit of open road) and that's assuming i've even made it into a high gear...

Has anyone else experienced this, and know of any possible cure?
I was told that the injector pump could be causing it, and duly sent it to be repaired, which made not the blindest bit of difference. I have recently changed the fuel filter also, but that didn't help, either.

Any help that could be offered is greatly appreciated.