s4 rough acceleration

["Martin, Gary G" <MartinGG@aetna.com>]

I have a 94 s4, bought at 87k,nearing 100k now and seems to have developed a
roughness during acceleration.
Most noticeable at 3000 to 4500 rpm at half to full throttle in second gear.
Less in 1st and 3rd. None in 4th and 5th.	
Idles fine, pulls strong. More noticeable when cold.
One shop couldn't find anything wrong, another had a computer read out of
failing engine RPM sensor, which was replaced and seemed
to improve it somewhat.  Any suggestions? 

94 S4..100K
87 5K CST..205K 
86 SAAB 9K...206k