Acceleration drop just before 4k rpm - Audi A4 2.0TFSI

I wonder if someone could shed some light on an issue that's developed on my 2005 A4.

When accelerating, just before 4000 rpm, the engine drops about 500 rpm and then accelerates as normal. Oh, and only happens in 3rd gear.

The only thing recently of note, is that I had the timing belt replaced a month ago and since then has suffered reduced mpg (around 27, down from mid 30's), but otherwise drives fine.

From what I've googled this could be mega bucks but noone has supplied a definitive explanation.

I hope the above sounds familiar to someone.

Thanks