Again, things it "could" be given the info provided:
#1 current draw up the grade caused the engine to find a weak spot in the track power system (lack of power feed, loose or dirty track pins). Engine just happens to be on the dead or intermittent power feed section.
#2 Something shorted or derailed in the curve and grade and the transformer or power supply is kicking in and out (self reset overload circuit). Again, grades can be problematic and the easement between the level flat and then the grade is where anything can hit and intermittently short to 3rd rail.
#3 Drawbar on a PS3 engine under load can become loose and intermittent connection
Now the bad stuff
#4 Let's say between current draw of the motor pulling up 22 cars up some steep grade and curve exceeded the current limit of the motor driver section of the boiler board inside the engine in a PS3.
#5 The drawbar coming partially unplugged under load causes certain pins of the 6 pin connection to open circuit while others are connected. This causes an incorrect voltage differential popping both the boiler board and/or tender PS3 board.
I would move the engine down to a flat level section, and preferably with a power source with meters for voltage and amperage draw slowly power up the engine after carefully checking and seating the drawbar connection. If the track has DCS and the engine won't respond to DCS, then attempt conventional testing using only a transformer and no TIU.