My 1989 420 has started running rough once it warms up for 4-5 minutes. Generally, when I first crank it up it is smooth, both at idle and when I accelerate. After it gets warm, the idle gets real rough, and it sputters when first starting to accelerate. Once you get 5-7 MPH it seems to smooth out and acceleration is fine. I noticed if I run it on the interstate and then get back in 'stop 'n go' traffic, it does not have the sputtering problem - it accelerates fine from a standing start. The plugs appear fine, no apparent hoses loose under the air filter shroud, the motor mounts were replaced not to long ago so they should be ok. Any thoughts would be appreciated. Thanks.