Q: Stalling at 50-55 mph

I own a super duty XLT 5 passenger van with 110,000 miles on it. Last month I did a full tune up - changed the spark plugs, fuel filter and air filter. I drove about 800 miles this month. Last week my van died while driving 50 mph on major highway. I lost power control, and it wouldn't start. When I turned the ignition it made an alarm sound - ti to ti to ti to - and where the odometer is it read “Rsc not working.” I had my van towed to a Ford specialist. The van started while the repairman checked it out. He left it on for a few hours and drove 20 miles with no problem. He said it’s a harness or wiring issue for sure, but he can’t figure out the problem until the van goes dead again. It died driving 80 more miles at 55 mph. The same alarm noise went off when I turned the ignition. The check engine and a few other warning lights came on after it died. The battery and lights worked fine. It started 10 minute later and ran like new then died after driving 2 miles. It started again 5 minute later but died after driving 3 miles. The next day it wouldn't start at all, so I had it towed it to the mechanic on Saturday. When he checked it on Monday he said the van started fine. I had him drive it for 4 days until he put 100 miles on it. Nothing went wrong. Then I put on another 350 miles last week. I don’t know what to do. Do you have any ideas or suggestions as how to solve this problem?