The style of lock up plate used in most torque converters are not able to hold large doses of torque. Also they apply quite smoothly (which Mum and Dad motorists desired) that accelerates wear. If your mildly hotting up your NA, either get the lock up plate to apply faster (drill 1 hole larger) or unplug soleniod. With bigger HP increases unplug soleniod.

Remember, torque convertors are cushions between the engine and gearbox. I think you'll find that the problem is the engine. In 1st to 3rd gears the engine and box seem to drive smooth, due to the fact that small changes in engine speed is cushioned by the convertor. Once the engine is locked to the driveline, any miss or problem such as the air flow meter giving dodgy signals will plum back onto your driveline and make your car shudder etc. This would always occur with mine when ever the engine was having probs. The car would shake to bits in 4th but in 3rd it would drive smooth, just the engine would run rough.