Several things can cause this condision.and heres the list:
(1) : Poor fuel quality,( the fuel could be contaminated with an unburnable substance "like water")
(2) : The engine could be in need of a tune up ( but first you need to detrimine if the engine is worthy of a tune-up / depending on the milage of the vehicle :
a compression test /and-or/ a cylinder leakage test maybe needed, Is the engine miss-firing? this could be caused by old faulty sparkplugs,ignition leads,or the fuel injectors, vacuum leaks can also cause a miss-fire condision when the engine is at idle , which will seem to disappear when the accellerator is raised off of idle.
(3) : If in the passed the engine has suffered from a faulty fuel injector,oxygen sensor,oil getting into the combustion chamber /and or/ an excess of unburnt fuel finding it's way into the exhaust system and to the catylitic converter this will cause the converter to clog. (which in turn restricts the exhaust flow and will impede the engines ability to breath) (Please note a engine is basically an air pump it pulls air in and pushes it out ) If you find out that the converter is clogged something caused it and your new converter will clog again if the problem is not found and repaired.
(4) : The problem could also be in the transmision: Check the fluid is it burnt? (Dark fluid with a foul odor is bad) (Good fluid has a bright red tint to it, and it smells like new oil)
If you would like a more detailed insight on what is going on I will need some more input from you: like milage on engine?, size of engine , type of transmission,What repairs have been done?,when did it start acting this way? does run ok when it's in park? any miss-fires, Does it only act up when a load is applied? my best guesses are printed above good luck - Mike
