Thanks to all who have shown interest in my problem. As I said, I have sent it in for a checkup, and included a number of the suggestions you guys made in a note I left in the car for the mechanic. So they checked it out as you suggested. Here is there report:
System Test and Diag Tech: 113 Engine stalls on highway. Checked vehicle operation for stalling on highway. Checked DTC found P1574 present but not related to stalling issue. Carried out checks as per customer request, checked plug leads and ignition coil connectors, all okay. Carried out diagnostic check on select monitor, all okay. Checked wiring connections at main harness connecter under bonnet and under dash, all okay. Road tested vehicle and checked all data, all okay. Was unable to reproduce fault at the time of testing. Have sent FPRS to Subaru regarding this issue.
Inspection Cost: $647.26
So it looks like I'm up shit creek. Does anyone know what P1574 refers to?
(Oh, I'm just joking about the inspection cost--there was no charge. :)