300 tdi overfueling, any ideas please?

Can anyone help?
My current disco's not been running right since I got it (cough, last april). Seems as though its either over fueling or not burning the fuel properly.
It idles too fast (turned it the throttle as far back as it'll go on the pump) smokes a heap on start and then occasionally when driving dumps masses of black smoke on top of another poor road user. It does that more often when on hills and towing.
I have some ideas what could be wrong but thought i'd ask about abit before I go throwing money at it. Any ideas?
The strange thing is that there's no logic to it. Somedays she runs really nice and others she's a bit moody. Reminds me....
Is there an electric choke on a 300tdi or could it be an electrical connection to thermostat/choke?

mine did that,but ok on start up,did it on hills in 2nd,blacked out most of Spain,
it was the egr valve,very common fault,fitted a new one, perfect!next time,
I`m blanking the thing off,try that first.

has plug on boost diaphragm on injection pump been tampered with,have you check air fillter pipes from filter to turbo arnet collapsing inside when turbo sucks air as you rev it ,pipes from turbo to cooler to manifold arent leaking ,waste gate not stuck open , before getting pump/injectors looked at