OK, here's the deal. I've never personally had to do a harness on a 119 engine before. I have heard others say they have, even here on this thread. My problems with harnesses have been pretty limited to 104 engines, from the first year of SFI (92?) up to 94, maybe a 95 or two. And with the 104's anyways, sometimes you'll have a driveability concern, but usually just check engine lights, and usually those have been related to EGR or AIR injection switch-over valves screwing up due to the broken insulation. But I don't admit about "knowing it all", so it's a definite possibility you can keep in mind.
If you can wait, I wouldn't mind looking at the info I have at work for these codes. Just so we're on the right track, the ONLY pertinent trouble codes are 17 and 18, and I am kind of assuming the codes are in the EA (electronic accelerator) control module. You don't have ANY codes in either HFM (engine control module) or DM (diagnostic module)?
I assume you have already paid for diagnosis. You could always say you'll go for the control module, and you'll pay for it if the car is definitely FIXED. If it's happened 4 times in the last month, tell then you'll pay for it if it goes 2 or 3 weeks without a fail. If it fails again, it's THEIR control module. How's that for diplomacy?