recurring E 030 error
 

I have a '91 Seville and replaced ISC motor to fix E030. Replaced ISC 2 additional times, have also replaced TPS, ECM, and distributor rotor. Continue to get same error. Checked for proper Cruise control function and replaced all vacuum lines around throttle body, can't find any binding in cables. Recently installed new muffler and a week later error became more persistent. RPM's are too aggressive in gear and in idle. High RPM's cause down shifting to thump. I can tap the accelerator at stop light and idle will often reduce to normal level. Replacing ISC motor fixes problem temporarily but is not addressing the issue. Any suggestions would be helpful.