false multiple cylinder misfire

'98 dakota, I get intermittent multiple misfire for cylinders 1,3,5. Mechanic says it is due to an small oil leak that is hitting a sensor. I am not confortable with his finding seeing that it was thought to be carbon after they did a motor vac. This started to occur after I had a lot of work done; tune up, replaced catilytic converter, motor vac gasgets replaced, etc. Truck runs fine although I get the check engine light for this.
Any thoughts of wisdom? Thanks.

Did he say which sensor?

Is the engine actually hesitating or can you hear the misfiring?

If so, I would lean more towards clogged injectors, or a loose connection in the injector system. Or something went haywire with the ignition system during the tuneup.

If not, then it sounds more like a bad sensor feeding incorrect information to the computer. However, I really don't know which sensor is used to determine whether or not a cylinder had a propper fire. Maybe someone else can chime in here.