ford mustang

I was helping a friend of mine fix his 98 Cobra Mustang. It has a 4.6 liter eng, he wanted to see how fast his car was so he jumped on it and the MIL light came on, I have a Autoxray code reader and found that he had a bank 1 runner malfunction. I cleared the code and cleaned the throttle body. The car idled smooth so we took it for a test run. We noticed that after we drove to about 40 mph and came to a stop the idle would increase and decrease + or - 1000 rpms then it would stablize to a normal idle. Has anyone encountered this problem? And about the runner we are going to pull the intake manifold and clean the runners, I hope that will fix the MIL problem.

Runner malfunction? What P code number is that, sounds like a vacucm leak or a leak in the intake air ductting, Might have an iac that's sticking or the Maf might requiring cleanning more thoroughly

Good luck and keep me postted

Jeff