Would be inclined to agree with Simon.
Rotten eggs was a typical effect when the first tanks, sorry Volvos, had cats fitted and owners put leaded petrol in by mistake. Cats got mangled and needed work at frightening cost. Mind you it took the smug smile off their faces.
Amonia is a more pungent smell than the fouled cats and can make your eyes water so it might be valuable to gauge the difference.
Other options certainly include slipping clutch but also brakes. The S1 had smaller brakes than the S2 onwards if I understand correctly. If you are driving harder to test your new RAM chip then you might be pushing the brakes harder, ditto the clutch.
Get someone with a sensitive nose to go from point to point before, during and after to see if you can pin it down.
Also try the hood up and see if it is coming in, close the vents be hand and switch off the fan. Be careful of overdosing on fumes if you are catching exhaust gases. I had to put a second layer of foamy stuff around my airbox intake under the bonnet to quell gases coming in through my ventilation/fan system.
I would also be very interested to get your feedback on the chip upgrade, I had been thinking about it for some time as well.
