Audi A3 TDI engine power loss while driving

Hi,

As Audi owners, as anyone come across the following fault? While I am driving the engine just drops into idle mode ( not always, only sometimes )  and if I put my foot down on the accelerator nothing happens. It engine doesn't rev it just idles but if I lift my foot off the accelerator and then back on it, it  works ok again. Or if I drop a gear the engine kicks in again. very strange fault. I've had it mechanically checked but nothing came up on the diagnostics. 

Can be a little scary at times. Any ideas? 

Thanks in advance

Remo.