2005 Volkswagen Fox losing power

I've had the car nearly a car now, a month after I first got it, it randomly lost power on me however I had no further problems with it, until recently. Occasionally when I start it, it judders and sounds like it is going to stall for about a minute and then it comes out of it and will then drive fine. However yesterday it did this and when I was driving home it kept losing power so I stopped in a lay-by. As I pulled over it sounded again like it was going to stall. I left the car off until my dad came and it started straight away with no issues, however as I was driving to the garage it just had no power in it, my foot was flat on the floor and the max I could get was 20MPH. My garage has put it on the diagnostics today and nothing came up and it drove fine for them. As I picked it up to take it to another garage, it felt like it was surging on me? any ideas what this is???