help at low revs power serge

hello can any one help when i start my car whichis a m reg tdi300 it ticks over well but then now and then it starts reving up and down only to about 1000 revs and if i drive slow just over 1000 reves it surges power not all the time but its geting more ofton.
i have changed the fuel up lift air filter and fuelfilter under bonnet and glow plugs
any ideas
thank you
john

could be a dodgy injector! i would try some injector cleaner or bio diesel to clean out the fuel system