I know you checked the timing, but did you also check the plug wire orientation?
I had the same thing happen after a top end re-build on a Misubishi engine. Turned out that in my haste to put things back together, I had the plug wires off by one, thus sending the spark 90 degrees out of sync. (Ah, the education we recieve) 30 mph, down-hill, in a hurricane with a tailwind was the best I could get, and the backfiring was horrible. Fixed the wire placement and I was back to crusin' 90 on the expressway!
This may not be close, but if the mechanic had the head off and did not pay attention to the wire placement......just a thought....
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles