Secondary oil cooler not functioning

[930gt-40r]

After sorting the engine out and actually driving my car I have noticed that the car has been running hot. At first, I thought maybe it was running too much timing and that is why it was getting hot... until I felt the secondary oil cooler. I had the car almost to the third notch on the oil temp gauge and the front oil cooler wasn't even warm. My first action was to change the secondary thermostat, but that didn't change anything. If i get the car to running temp and then rev it to about 4000 rpm, then it flows the oil through, but this only happens sometimes. For the most part, the secondary oil cooler is cold as Hagen Das. Any ideas anyone?

P.s I doubt there is a blockage because it does let some oil through at some times and I know that the last owner was nuts about keeping the oil clean.