I had the exact same
problem about 4 months ago, on my 1.2
ibiza. Whenever i got to 3000rpm the car started slowing down. It seemed to fix itself once i turned the engine off and back on though and only happened that one time.
Ive been getting a squeaking noise from the car since around then and its been getting worse and worse. it happens whenever i take my foot off the accelerator pedal, it also happens when i'm on rough ground. Dont think its the suspension though as doesnt happen when the engine is off and i bounce the car... wondering now if the two might be related? Anyone any idea?