Hi I have a 2001 Xterra, Manual, 213,000, runs rough at random
If your mechanic changed the spark plugs. Make sure he did a few things.
First make sure he used NGK plugs. I have seen other make plugs cause running conditions.
The other thing is make sure plug number 6 was changed. I can not tell you how many times I have seen shops leave that one cause it's to hard. That also can give you the ruff condition.
Last but not least, if the distributor cap and rotor and wires were not replaced, it may be possible that you could have a spark issue being loss. If you notice it more when conditions are wet, then it has to be related to those items.