Fuel Pressure, Lean mixture, pinging in Volvo 144 B20E

Hello Chris! 
Seems like a hard one to solve.
Wonder if you have checked the voltage at the fuel pump terminals exactly when fail occurs ?
Also a tip, the coolant temp sensor can short to gnd or give very faulty value when it is warmed up (even if ok at cold state), i have that experience. This can make engine run rough or even die. Exclude as failsource by putting in one resistor in the two-pin connector, simulating the temp of your engine (See manual for correct value on resistor)

Know that the last one is not related to dropping fuel pressure, but just saying.

Another tip is that the injectors and even cold start valve(leaking) may cause problem when engine hot. Have seen that. May be excluded as fal source by a plier, one and one hose blocking, monitor pressure gauge while blockin one and one hose.

Anyway, hope you and your volvo is ok

BR Terje