Sounds like you have a lot of fun. Look on the diesel forum too for info. Lots of experience around. You might have a kickdown switch issue also. Take up the floor mat and unplug the kickdown switch. If it now upshifts that was the problem.

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My Daily : 96 E-300 Diesel with 195,000 miles
Retired: 92 300D 2.5 T 345K miles and for sale
Retired: 95 E320 157K miles and currently parked with blown engine

Both retired cars are for sale as is my w124 shop inventory