The other possibility is a oil pressure/fuel pump switch that is going bad but hasn't completely failed yet. If the resistance is high on the oil switch it will slow down the pump reducing its pumping efficiency.
Can you use your primer button to bypass the pressure switch with the engine running. If so when you have low flow, energize the pump with your prime switch,if you get increased flow then that would verify a bad pressure switch.

If it is the switch it will progressively get worse, until it won't work at all. When i put transfer pumps on i energize the pump through a relay that is operated by the switch rather then using the switch to drive the pump.

good luck