Hello!
I have a 2010 E350 v8 (92k) that misfires at 40 mph. I brought the van in for a tuneup (oil, filter,pcv, plugs) The van ran perfect for 3 weeks and then the misfiring came right back! My mechanic gave me a bottle of fuel injector cleaner but that didn't work either. Engine is saying random misfire. What do I do?

 

There's a decent chance your catalytic converters are bad. Disconnect them at the engine and look down in the exhaust pipes.
Pretty common problem here, and very similar symptoms

 

Have you checked for DTC's yet? If so what were or are they?