I have a 1996 SL500 that has a random misfire on cylinders 5,6,7, and 8. I have done the manifold lower gaskets and the donut gaskets, all injector seals, replaced the MAF, ATS, and all plugs. I was able to see the random misfire on my scan tool. When the car is first started cold, the idle is fine. After it runs for a few minutes it starts to stumble at idle. When I bring the idle above about 1500 the random misfire goes away. I can visually see this on the scan tool. When I let the car come back to idle the stumble returns. The stumble appears to be getting worse with time but has yet to make the car stall.

Open to any suggestions. Thanks.