A stumble at highway speeds is often related to *speed*. I would check the speed/abs sensors at each wheel for debris or improper spacing from the tone ring.
No, I take that back. I would FIRST check the tire pressures. If they are particularly low, or if one is significantly lower than the others, it can cause wonky readings in the ABS/ASR systems and the electronics would take over, thinking you were either spinning out a tire when you weren't or that maybe you were hydroplaning and one tire is moving signifcantly slower than the other three. When the ABS/ASR systems activate, they often electronically drop the rpms, making you feel a "stutter".
Same thing with the sensor/tone ring interface. If its off or something is not reading correctly, the ECU can think you are skidding or hydroplaning and take what it thinks is appropriate action. These "stutters" would also NOT be accompanied by a MIL (CEL) because the ECU treats it as a normal occurance of driving. Otherwise the MIL will illuminate every time the ABS kicks in.
My sister in law called me last winter a few times saying that her sprinter kept randomly losing power when she was driving on the highway. Then it would come back again. When she finally came by, it turned out to be a low tire. The high speeds and low air pressure deformed the tire enough to make the wheel spin at a different enough speed for the ASR system to kick in and the ECU to limit power for a second. Then everything would go back to normal.
While the brake light switch is part of the ABS system, I am not aware that any signal from it is necessary unless you are braking. Therefore, a "stutter" at highway speeds is probably not related to the brake light switch... Unless, of course, you had a short in the brake light circuit and a signal was being (reversley) sent from the light up through the brake light switch. But why that would only happen at highway speeds is beyond me.
