Thanks for your post. I believe its the main timing belt. Its possible there's more then one problem. The main symptom is the engine keeps on missing with a lot of smoke from the exaust. this has been happening for 6 months now with no other problems.
Lets look at what the timing belt does.
It maintains the critical timing between the valves opening & closing in conjuntion with the pistons running up & down the cylinder bores. This is a finely tuned process which if the belt slips the valves will hit the pistons...
If this is happening regularly the following must be true.
1) You have been extremely lucky...
2) The belt must be slack...
3) One of the elements responsible for maintaining the tension is damaged.
4) The timing is not set up correctly...
For #1 above, that is amazing...
#2, 3 & 4 should have been spotted by who-ever re-does the timing.
Personally I'd take the vehicle to someone else that can be trusted.
Suspect the MAF sensor, EGR or possibly a Turbo related problem.