Unexpected surging revs - can anyone help
 

Am a new Audi convert with a A4 2005 Avant. I am really puzzled - when idling the rev counter surged dramatically and when travelling at constant 100kms @ 2000 revs the rev counter surged up to 5000 revs. I then deaccelerated and it dropped back down immediately. The car is also very sluggish when first accelerating and will drop off in power as soon as I take my foot off the accelerator. Would really appreciate any advice