Sounds like your head gasket is leaking compressed gases from a cylinder into your cooling system (this is the bubbles).
Once the engine is warmed up the hot water in the cooling system is pressurised, while the engine is running at a medium revs or above cylinder-compression and coolant (hot) pressure are equalised so the counter-act each other.
But when the revs are at tick-over cylinder pressure is lower so a small amount of water is pushed into the cylinder and exhausted out creating whit smoke (ie; steam & smoke mix) remember that one small drop of water when heated expands by at least 1,000 times volume!
A temporary remedy is to purchase some BARRS Sealer (a few pounds) and put it into the coolant header tank and run the engine for about 10 minutes then do a few miles down the road. This I have done myself and has kept me going until the head gasket was replaced.
The juddering when accelerating is small amounts of water in one or more cylinders wetting the spark-plug preventing it from firing on that cylinder. Another symptom is after the engine has cooled off overnight the next morning on starting the engine will mis-fire for a while until the moisture leaked from the cooling system is cleared out by engine rotation.
Note: If you change the head gasket have the head checked for warping as if this is so then suspect the radiator thermo switch as being faulty because this does not switch on the radiator FAN when the engine gets hot, then overheating will occurr thereby warping the head again. You might as well change it when changing the gasket as you have to drain the coolant out anyhow (the thermoswitch is only about £10.00) to stop it happening again.
Another reason for the head gasket to leak is by corrosion this happens if there is little or no antifreeze mix in the coolant, it has to be kept in the ratio of at least 1 x part antifreeze to 3 x parts water, as antifreeze has a corrosion inhibitor that prevents electrolytic action between the different metals in the coolant system eating the gaskets. Antifreeze should be kept at its optimim strength in the engine all year round to prevent this. I have seen this problem many times during my carreer of 30 odd years.
Mike
April 2007