Sluggish car

Can anyone please help.

I have recently purchased a Skoda Superb 05 plate. It as only done 10,000 miles. It has been running great but over the past 2 weeks it seems quite sluggish and doesnt have the usual power. Also noticed occassionally that whilst driving it splutters and jumps then its as if someone has pressed a button and its fine again.

Its at the garage now and being told the diagnostic is saying there is nothing wrong.

Any ideas anyone????