fastd Phil it's a heat soak problem. I have had this problem years past, 15 minutes and the trans starts to not want to shift. The fluid also gets warm for the clutch. I doubt that the clutch fluid is boiling but if your fluids only have one little tank for both brakes and clutch this COULD be a small problem.
Here's what I have gone through over the years and ended up with
If you have inward rear brakes, there is alot of heat generated from them. Also the same applies for the motor and trans in the trans tunnel. This air has not where to go and stays there. I have an added fire shield on my tunnel just in case. You have 2 choices for a fix,
Add a cooler, pump and fan kit for the trans fluid and run it at the track only times. this is about a $600.00 kit plus lines and connectors.
The idea I use is to add 3 bildge fans under the hood to remove the air. I use the r/s air hose from the front to a 3" blidge fans mounts on the r/s foot box to blow air down the tunnel. A small hose comes off the back side. This work well with fresh air outside of the car. I also added 2 more 4" , 1 on each side of the motor compartment to well remove the hot air under the hood. They blow the air out the louvers in the fenders. You need to do a little wiring, couple of relays, and switches. Each fan pulls about 3-8 amps demending on heat. Here's the other problem with 3 fans you will not have enough charging ampage to run everything in the car and these 3 fans. Need to go to a min of a 95 amp alt, and change over the amp gauge to a voltage gauge on the dash. running this much ampage through an amp gauage will cook the insolator on the gauge over time. This happened to me. P-M for more info. This fixed my over heating problem. Rick L. Ps I am still going to run a rearend
oil cooler in the car.