I have a 92 convertable that has an eratic idle. Sometimes it is hard to start and idles at 500 to 600 rpm. Once it warms up the idle smooths out at about 900 rpm. Then sometimes you start it and it goes into a high idle and will not knock down. When it is warmed up she races at 1800 rpm. Hard to hold at a light unless I drop the auto trans into N. The local dealer wants to flush out the plenum because he says the idle motor is sticking. There are 50,000 miles on the clock. Any ideas or sage wisdom out there.