Misfiring when idle, no engine light

I bought my car (09 VW Beetle) just over a month ago but have noticed that it has a rough idle. I took it to a mechanic who said it has a misfire on cylinder two and four, but the diagnostic machine isn’t picking up any errors. They advised that I come back as soon as the engine light comes on. They said they know what the issue is but wouldn’t be able to identify the part causing the issue due to their being no error code. The rough idol is seeming to get worse and I would rather find a solution even if it cost more money in the long run. I also don’t want to be driving the car with the issue persisting as I don’t wanna cause further damage to the engine. I thought about trying the cheaper fixes first as a first step. I would be very grateful if anyone could advise me on what parts are most likely needing to be replaced/where is a good place to start? Just to note, I’ve had the spark plugs changed recently in a service.