This might be a long-shot, but someone had posted that they also had a weird hesitation problem, went through a long list of parts and repairs, time and money, but it turned out to be the ceramic insulator behind the rotor, under the distributor cap.
Now how anyone would ever know to check this, amazes me. My guess is that it was cracked and allowed the coil spark to ground out to the head.
Since your problem is worse as the engine gets warmer, it may follow that a tiny hair line crack expands as it gets warmer.
May be worth it to check it out. Only takes a few minutes.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)