Need HELP. Tranns cooler and shifting problems

I added a transmission cooler on Saturday. Everything was fine for about 150 miles and now my tranns is acting up. Everything is smooth up too 30mph. Once I hit 30 and I am very lightly accelerating the RPM's are around 1100 and then it happens. The truck will kick and vibrate. Like it is bogging down, but the power never goes away. Once I am above 35 mph the vibration and noise goes away and all is well.

I tried this several times on gradual upward grades and it did it 8 out of 10 times at the same speeds. I tried to do it with the overdrive locked out and it did NOT do it.

I installed a b&m cooler. Ran the return line from the factory radiator cooler to the input on the b&m. Ran the return line on the b&m to the return line on the transmission. All the lines only have a very gradual sweep to them, no tight turns. I added the 1/2 qt of toyota WS fluid as recommended by b&m. Everything should be fine.

Anyone have any suggestions what it could be? I'm all ears.

THANKS!