intermittent white smoke on ford transit?

Hi,

My Ford Transit LWB 2.5 Turbo Diesel P reg will drive fine for several thousand miles, then on a random occasion I will start it up and it will smoke massively from the exhaust - so much you really can't drive it because you leave a trail of white smog down the road behind you.

If you switch it off and start it again, half the time it will start fine without smoke. The other half the time you have to switch it off and again several times over a period of a couple of days til the smoke goes away and it runs fine again. It always stops doing it eventually, and drives really well the rest of the time.

It doesn't seem to lose power. It usually happens on starting it, not in the middle of a journey. The smoke is definitely white not blue, and is very smelly (although you might get a little bit of blue smoke at the end of an episode.)

We have had new injectors, replaced fuel filters, had EGR valve fixed. Its hard to get fixed cos hard to get to a garage when it is smoking, and impossible to diagnose when it is not. Last garage said we should try replacing the fuel pump but that is very expensive, and no guarantee it will fix it (the injectors didn't make any difference)

Any suggestions on what else it might be, or how to diagnose, or how to make it go away when it happens? Thanks!!