I would also like to chime in and say that a motor rated at 3600 Max RPM should have no problem at 3100. Normal cruising RPM should be around 2900; 2250 is under-reving the motor.
In addition to the other thoughts - you may want to check your hoses. As they age, the suction created by the higher pressure at higher RPM's can cause them to restrict flow. If the hoses are older, replace them as part of normal maintenance and you may see an immediate improvement.
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