The cycling is the result of the diameter of the drain pvc pipe you are using. As the water is draining down the pvc pipe, it starts building up back pressure due to the 90 degree elbows and the 6’ horizontal run. This is causing the over flow to not be able to keep up with the water that is returning to your tank by the T3. As the water level rises in your overflow, the water weight increases the preasure enough to overcome the back pressure and the overflow water level drops. This cycle keeps repeating. Is there any way you could add a second drain line or increase the diameter of the original drain line? Also, is the drain line output submerged?