Funny oil pressure on 330 GTC

I am having problems with the oil pressure on my 330 GTC. After the engine is at normal operating temperatures, if I am running the engine at a steady 4000 rpm, the oil pressure is at 5 kg/cm2. If I step on the gas, the oil pressure goes DOWN to about 3 kg/cm2 for several seconds. After getting to about 4500 rpm the oil pressure goes up to 6 kg/cm2. If I return to a steady 4000 rpm, the oil pressure returns to 5 kg/cm2. If I release the gas, the oil pressure momentarily goes UP to about 6 kg/cm2 for several seconds.

Normally I expect the oil pressure to go up at higher rpm and the oil pressure to go down at lower rpm. What can be wrong?

Thanks,

Enzo