Oldsmobile - 88 :: 1997 - Intermittent Hesitation Speed At Highway Speed

I have a 1997 Oldsmobile 88 (174K miles, owned only by my father-in-law and then me) with which I am experiencing a hesitation problem from time to time at highway speed. My mechanic has taken the car on several highway test drives throughout the last two days, and he has been unable to reproduce the problem. Furthermore, he has done a full diagnostic test and found no problems with the fuel system, ignition system, or anything else.

Here are the conditions under which the problem occurs:

---Attempting to accelerate gradually at highway speed (usually from 50-60 mph), but WITHOUT pushing the gas pedal down so far as to cause the transmission to downshift
---Engine fully warmed up (temp gauge at 200 F or just below)
---Occurs more often when I have been driving for at least 1-2 hours

The condition is as follows:

---Car lurches/surges noticeably and does not accelerate
---Speedometer needle reflects the lurching by fluctuating up and down about 1 to 2 mph
---Accelerates well if I step down hard enough to cause the transmission to downshift, or stops surging if I lift off the gas pedal
---NO Check Engine light at any time

Some history on the car and corrective actions taken:

---June 2009: My mother-in-law was driving the car about 6 hours into our trip and it consistently hesitated between 50 and 55 mph with the Check Engine light on for the last 30 miles or so of our trip. The next day, we took the car to the dealer where my in-laws had their cars serviced (note the past tense here), where they told us that "there's probably something wrong inside the transmission" and suggested that we replace the transmission with a new one. This transmission is the second new one the car has had (replaced the first time at 95K miles), meaning that this transmission had less than 80K miles at the time. We took the car to a local shop at the referral of a friend, and this mechanic suggested we try flushing the transmission fluid and replacing the filter to see if that would fix the problem before we started talking about replacing the whole transmission. This worked, and we were glad that we did not have to get rid of the car. Did not notice any more problems with this for over a year.
---May 2010: Experienced engine misfire, had spark plug wires replaced. Problem solved.
---March 2011: First noticed my current problem. Had fuel filter replaced (it was probably a little overdue) and had the mechanics do a fuel system cleaning.
---April 2011: Problem still occurs.
---Had throttle body thoroughly cleaned, which fixed a slight sticking in the gas pedal.
---Replaced spark plugs myself. Car running more smoothly overall, but the hesitation still occurs.
---Inspected all three ignition coil packs myself. Top coil pack had some rust on the poles, which caused the resistance to measure as zero. Cleaned most of the rust off the poles, after which all three packs bench tested at 6.23 (with 20000 ohm setting on multimeter). Car runs even more smoothly overall, but hesitation persists.

At this point, my mechanic and I are both stumped as to what could be causing the hesitation problem. My only thought right now is to try replacing the one coil pack that had rust on it, since a pack with a fault I can see may have others inside that I can't see, and maybe something like this is flaking out on an intermittent basis.