2006 Subaru Forester (SOHC)

I have a regular mechanic that I use, except for a recent plug change. The price was twice what I was quoted at another local mechanic, so I decided to save some money. The car now feels like it is shifting awkwardly, and at speeds of between 15-25 MPH, it’s as if it has a hard time choosing a gear. Can improperly installed plugs have an impact on this sort of thing?

Great rule of thumb:

If the vehicle was working fine, but then suddenly changes in a negative way after work is done to it, it most likely was the work that was done’s fault for the change.

Maybe the spark plugs aren’t the right type.
Maybe one spark plug wire isn’t snapped on good and tight.
Were the spark plug wires changed too, or were they reused?
If they were reused, then its possible one of the wires is bad now.