According to my code discriptions from Startek info code P1617 is listed as "Control module or not coded" It almost soounds as though your control module is temporarily loosing it's memory. Just a guess. It also makes a reference under test step - remedy, see HHT (hand held tester) "version coding" menu selection 5. If you get it to a MB shop your may want to show them this. Hope it helps.

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Keith Schuster
2006 E350 98K miles
2013 Ford Explorer 15K miles