I don't like the sound of where and what your FSD is mounted on. Take a look at some of the Kits and heatsinks available and compare. You might want to wait for more appropriate advice but sometimes the stainless steel screws on the FSD loose contact. You might try removing the FSD from the sttel plate and carefully removing the caps off of the two T03 case transistors. Then very carefully loosen and tighten the 4 screws - Not too tight!(2 for each transistor.) Then use some heatsink compund or pad and remount.
After reading all about FSD relocation and another alternative unit, I still believe the FSD to be the way to go and the best place is on the side of the IP where the manufacturer intended it to go.
The problem is that most time the damage occurs after the engine is switched off when the heat builds up in the engine bay. Whilst the engine is running the fuel running through the IP keeps the FSD cool but after the engine stops the heat build up can cause damage. For this reason I can't see a benefit to the Repositioning of the FSD to a Heatsink in the Enginebay except for the ease of replacement when it too fails.
I haven't seen your installation, but I'm not sure I would put a steel plate in the same class as a heatsink.
For the vehicle to have been running fine when you switched it off tends to lead me to think the FSD is worth a thought!
Noel
1998 (Badge Re-engineered)RHD GM-Holden Chevrolet Suburban K2500, 150k miles, Turbo Diesel 583Nm 200Hp, New 4L80E, 3.73:1, Rebulit T/case with New Housing, standard 245's factory 6.5" wheels. No Mods. Car Code OBDII interface with Enhanced GM data, test & command modes, data logging, live data etc.
Toyota HZJ80 Landcruiser 4.2L Turbo Diesel, 195k miles, 430Nm 210Hp. Mitsubishi Water Cooled Turbo, Through bonnet Intercooler, Advanced timing and increased fuelling, 12 psi boost, Boost gauge and post Turbo pyrometer. 5 Spd Manual.