Hope this all makes sense.

I drive 2013 Fabia VRS, have had this problem for a while and it'll only happen periodically. 

So basically the engine will rev up and down, only to around 1200rpm before dropping but can happen for a while. When idle i'll leave for a bit and it'll calm down but when driving it can be an issue as it will sometimes cause the ECU light to come on and makes holding speed more difficult i will sometimes have to drop it down a gear.

Has anyone else experienced this? 

Need the fault codes reading to be honest before any "guesswork" on whats wrong, could be numerous things.

 

When fitting an uprated intercooler mines done similar, its would pop from 5th down to 4th when the foot was planted. I found the cause to be a boost leak.