Ok, firstly the fpr and frps are different things. You're reading fuel pressure in the high pressure rail with the FRPS (Fuel Rail Pressure Sensor) and regulating the high pressure pump output with the FPR (Fuel Pressure Regulator).
It does seem as if the fpr could be causing the problem, try blasting it out with a bit of wd40 or something (remove the o-rings first if using something like carb cleaner), that's a much better way of cleaning them. Worth having a look in the bowl of the fuel filter housing for any muck, water or swarf, just for an idea of what's come through/from the lift pump, in case that's causing problems.
Tbh with the high-pressure system at that age it could be the pump or injectors, certainly worth doing a bit more research before you spend a lot of money on it imo.
It does seem as if the fpr could be causing the problem, try blasting it out with a bit of wd40 or something (remove the o-rings first if using something like carb cleaner), that's a much better way of cleaning them. Worth having a look in the bowl of the fuel filter housing for any muck, water or swarf, just for an idea of what's come through/from the lift pump, in case that's causing problems.
Tbh with the high-pressure system at that age it could be the pump or injectors, certainly worth doing a bit more research before you spend a lot of money on it imo.