Had a similar episode on the Sharan (oiler) on Snake Pass! Went slower and slower until I thought we weren't gonna make it. Got to the destination, just, (the other side's all downhil!), unloaded the son's uni gear, started up again thinking "this should be interesting" and LO, everything normal. Got back (empty, obv) in no time. My Dub specialist identified it as the mass air flow meter - the wire sensor bends over time, callking for less fuel. Switching off and on resets the calibration. Those HT leads don't last long, though - the Lupo's EM light came on in Liverpool and we misfired all the way back to Southport (20 miles) with no other issue cos of one of them.