Let'* put the fuel pressure gauge down.
Recently we have a couple of members that fought this code in every manner possible.
Dyno...did you pull the booties off the battery connection wires? Also..you need to get under the car, remove the plastic shield and clean connections for the ground at the block. It looks tough but goes pretty well. Then pull the cover off the maxifuse relay center...and clean the connections where the positive lead is bolted to supply power.
The light and PCM seem to drive you nuts because it takes two consecutive drive cycles with a failure to come on and 3 good consecutive cycles to go off.
Here'* my advice, and take this from someone that went crazy like you are. Stop testing fuel pressure. If you want that light gone, hop over to the thread Lash linked, read it..you will see that 3-4 cars with this issue were cleared up with some form of battery cable cleaning or replacement of cable or battery. (battery was only replaced by San Juan Cruiser due to crack by terminal). I replaced cables because at that point..I was no longer playing around.
FP on a S2 SC is 48-57psi per the correct testing procedure of key on, bleed air, key off, back on ..off back on to attain max pressure....leave like that for 10 min to look for any leak down.