1.) 8v or 16v? Most common fault for stalling when slowing down to idle revs is the Idle Valve. Looks like a metal cylinder with 2 vacuum pipes and a electronic plug connection. If you have a 16v like me, its on the RH side of the engine near the dizzy. Anyways, it gets blocked up with oil and needs a good clean with brake/carb cleaner first (spray into the vac holes until no more black oily stuff comes out) if the idle still drops you MAY need another one or else you might have something more in depth that causes your stalling.
2.) Is the car overheating? You said that the light comes on after an hour filling it, are you loosing water? Could be the cap/rad/or metal water pipe spraying coolant over something hot and drying up. Check for white stains on the block if you are loosing water.
3.) With engine running and not moving can you hear the tapping? Most likely tappets as mattneck queried, but if its when you are on the move and possibly turning corners then it could be your CV/Driveshafts.
