Audi A6

Hello, first timer here, appreciate the car may be rare in the US, but here goes; I drive an Audi A6 Avant, 2.0l, 8 valve, auto, (engine code ABK). It's a 1996 model, with 96,000 miles on the clock. Basically, the problem is an occasional stutter/misfire/power loss, usually after a good hour or two's run, and more so when engine has been at high temperature for a while (after driving in town traffic). When you then get to an open road, the problem appears. There are no overheating issues, new coolant, no leaks. I have replaced the fuel filter, rotor arm, distributor cap, leads, spark plugs, air & oil filters, and most recently, the catalytic converter. These changes have vastly and gradually improved the problem, especially the cat change. Could changing the oxygen (lambda) sensor help, do I need to suspect the ignition coil or injectors? There are no fault codes registered, and for 99% of the time, the car runs super-smooth and accelerates well. Thank you for any guidance, I'm running out of ideas! Ron.