I've got a 1998 Savana 3500 with the 6.5 in it. When I start it the engine surges up and then it idles fine for a minute then it starts to run rough. After it warms up it runs fine until I drive over 80 kms hour. If I try to drive any faster the throttle opens right up as though I'm flooring it. Driving under 80 kms hour isn't a problem and it idles perfectly but only after it completely warms up.

I did the PMD last summer and wondering if it's defective or could the fuel pump be on it's way out?

 

Seems like a PMD to me. I had this happen too and it wouldn't stop the runaway unless I shut off the ignition. A new PMD solved the problem.

 

Brought it to my local diesel shop they put the scanner on it and took it out for drive. Not a single code and he checked all the values and they were all perfect. Technically the van should be running perfectly. Unable to test the PMD he suggested that it's probably the cause.