First thing's first. You're not feeling a "gear shift" what you're feeling is the torque converter lockout engaging and disengaging. A couple things caught my attention. It only does it when it's cooler out mainly. The torque converter will not lockout unless the engine is properly warmed up. You may simply need to replace your thermostat to fix the problem. With a 195 degree thermostat, your engine shouldn't be down to 150 unless you're just starting it. Since it's such a simple fix, I'd try that first. The same thing happened to me one December (I live in WI, where it does get 'cold') and I thought that there was a tranny issue as well, but all was ok afterward.