Just winging it here, but as changing the filter makes a difference the problem is very close to it. Take the filter adapter off the block . Check the gasket for tears. Check both the block and adapter for cracks scores and flatness. Take a mirror and look in the passages to see if there is an obstuction in there. The adapter is not an expensive item, just replace it. Usually comes with a gasket. Myself, before I went to all that trouble, I would take the sender out, screw a pipe nipple in there witha piece of hose and a can to catch the oil. Start the motor , let it run till you have pumped a quart of oil out shut it off and put the sender back in. The purpose of this exercise is to flush out that piece of crap that is plugging the sender. Changing the filter or shutting the motor off would let the weight of the oil pull it back away from the sender. As there is practically no oil flow there, it would take a while for it to work its way back up to the sender.
Thats all I got.