I installed an aftermarket trans cooler on my 09 Tahoe this weekend. 5.3, 6-speed trans. The cooler is a Derale plate/fin. After completing the install and checking for leaks, I took it on a quick 3-4 mile test drive and everything seemed to run smoothly. I did not add any fluid at this time (probably should have). Later that evening, we went out on a longer drive and I started to notice a vibration or fluctuation that felt almost like an engine misfire, but I have to assume it's related to the transmission (too odd a coincidence otherwise). It only does it under normal acceleration, if I accelerated hard, I didn't feel it any longer. but under normal acceleration, it shifts at the expected points, but after each shift, it shudders or vibrates, and I can see the tach needle jumping up an down 100-200 rpm's. I took it home and checked the fluid level hot and it was reading low, which is to be expected since the cooler and additional lines would add capacity to the system. I added a quart of fluid (seemed like a lot) to get it to the appropriate "hot" level. However, the problem persists. At this point it was late so i gave up. Are these symptoms something that could be caused by a fluid issue? Could it be something as simple as a hose kink, restricting fluid flow back to the trans and causing low fluid like behavior? Could the cooler be restricting proper fluid flow? This evening I'm going to check for hose links and if I don't find that I'm going to bypass the cooler altogether and see if the problem goes away.