Diesel torque peak lies at around 2000rpm so it might be clutch slipping only if engine speed increases but there is a loss in acceleration(speedometer stalls momentarily). The ASR "kicking in" (as stated earlier) might be the case also since if you normally tend to have wheelspin at third gear, the dip would mean that ASR is cutting power to prevent spinning.. this might depend on situation, sometimes if road conditions allow and the ASR may not activate even if it's turned on.. If the dip comes with engine speed dropping (even with ASR off) then you might have a MAF-problem or other malfunction with the ecu. Hope this helps..

When you look at the Revo curve it may well be clutch slippage, it gets a huge kick of torque in that range, does it do it in 4th and 5th as well.

You can find out by just planting it and looking at dash. If the rev counter goes up but your speed hardly increases and then 'suddenly' they start to both go up again, you've got slippage

I don't know if this applies to skoda/polo/ibiza but in vw golf pd130 engines they've fixed the slipping problems with VR6 pressure plate (the clutch itself can handle the torque but pressure plate is not stiff enought to keep it together).. the engine is basically the same so why not? .. maybe someone else has more information about this..? I currently drive 1.4 tdi polo myself so I haven't bothered finding out but my next car might very well be fabia rs so I'm interested..

btw, the slipping problem is very common with these engines in heavier cars when chipped up to 190bhp.. the reason it might not appear at first is that fabia is lighter in weight than golf and bigger cars.. but try to tow a trailer and see what I mean