Subaru Timing problem

My mechanic is stumped. My 1998 Subaru has a problem with “bucking” at highway speeds (kind of like when the clutch is let out too slowly). When it gets like this, it’s also difficult to shift (I thought that it was the clutch, but no). The timing belt has been replaced & the tensioner adjusted, and the spark plug replaced, and the car runs okay for a month or so, then starts acting up. Any suggestions?

Was the timing belt replacement before or after the problem started? Why did not I see that the spark plug wires were replaced?