My wife's 2004 1.6 Sport is becoming really difficult to drive smoothly, its almost impossible to do a smooth gearchange, it seems that when the rev's drop off there is a flat spot as you feed the gas back in. Its not due a service for 6 months and has only done 6k since the last one.

I have also noticed that when she drove off this morning that it smelt quite 'rich'.
It idles fairly smoothly at about 800rpm, but when switching the climate to Econ that drops back another 100rpm and becomes even more difficult to drive.

Anyone got any ideas? Would it be worth changing the plugs or air filter?

Thanks in advance!