Foaming C4 Transmission Fluid

[RED88GT]

I'm running a C4 trans behind a modified 306. My problem is that I have aerated trans fluid showing on the dipstick. The tranny was just rebuilt. The tranny has a 10" converter @ 2800rpm stall with a manual/automatic valve body. I have a 22,000 GVW trans cooler. I'm running original Trick Shift fluid (not synthetic). I have a drain pan with plug so I've adjusted fluid levels to make sure its not overfilled. I've checked the trans pump by disconnecting the return line from the trans cooler. No aeration in the fluid but I did this with a cold engine. I did check the temperature of the hard lines to and from the trans at normal operating temp. Trans line out temp was 135F, return line temp was 125F. So to me it looks like the fluid level is ok, pump is ok, and temps are ok. Also, even with the aerated fluid, the trans shifts ok and the car runs ok even after a couple hours of driving. Anyone have any suggestions as to the cause of the problem?