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Very typical. You've found the sour spot where you should try not to drive - max torque load with a less than torquey moter (I'm assuming 3.1 or 3.4 Chevy ). The "corrrect" answer is to not do this, if you have a long drive hold it in 3, or go faster or slower. But, assuming it didn't always do it, there are 3 distinct possibilities. note that htis is right at a double shift point which may or may not be a "shift". The torque converter lockup is not the same as the 3-4 shift. At 40mph under a load you are on the edge of converter unlock, 4-3 downshift with converter remaining locked, or both. 1. It is possible that the converter is not unlocking properly. A quick test - at 55mph on a flat road, in OD, the converter should be locked. set the cruise control. and note the engine RPM (tach or sound). When you touch the brake pedal lightly (to trigger the switches - do not brake the car), the switches will activate to cancel the cruise and unlock the converter. There will be a slight rise in engine RPM. Yes? then the converter is unlocking OK. 2. The eninge may have a miss under lugging load. Under the chugging condition, lightly touch the brake (again, to trigger the switch, not to brake the car). If the problem goes away without the trans shifting, suspect engine miss (dirty injector, plugs wires worn cam intake leak (very common on high mileage 3.4's) etc.). This is the most common problem. 3. the trans may not be downshifting properly. At 40mph my van is in 3rd not 4th, 4th is 42mph and tcc is 45. You'd have to have another car to compare with to be sure.
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