We've seen lots of posts about a certain kind of surging in various 3800 Series 2+4T60 platforms. I'm throwing my experience out there to help out.

My 1997 LeSabre started doing this a few years ago. The symptom was a slight surging on flat road at 55 to 70MPH . The frequency of the surging was about two surges per second. The tachometer would alternate around 100RPM concurrent with the surges.

Watching the tach it appeared that each perceived "surge" coincided with RPMs dropping about 100RPM, then the tach would jump back up 100RPM again.

Give it a little more gas and it would stop. Give it a slight bit less and the severity of the symptoms would diminish. Slightly less still and no more surging (but now you're almost coasting). Never a hiccup of anything unbecoming from the engine otherwise. Seemed transmissionny to me. Like slightly depressing the clutch on a manual transmission over and over.

For the longest time, all the interweb had solutions that were all over the place, often focusing on engine issues. Not saying some of them weren't, as the nature of the symptoms led me there first as well. At the same time the described symptoms seemed very similar to mine, and the vehicles were about the same demographic: 1996-1998ish LeSabres and Ninety-Eights with a smattering of Bonnevilles, Park Avenues, Regals, Grand Prixs, Luminas, and Impalas and such.

So, without a solution in-hand and with the only sure transmission fix being a full overhaul, I decided it would be the same overhaul when something fails more and later. In the meantime I let my mental autopilot keep the symptoms to a minimum. This was around 130,000 miles.

Then one year in winter it occurred to me that I never felt the surging unless I accelerated hard a couple of times in a row. Could it be temperature? Then as the weather warmed up in spring and I had this theory in mind I noticed that it was easier to provoke, and in summer took nearly zero provocation.

At the same time the interwebs were starting to talk about the TCC solenoid, which is in the side pan therefore quite a deal to get to on a guess and a hope.

Following my temperature theory, I put a "big" transmission cooler on it. Hayden model 404, 15.5" wide, 7.5" tall, 0.75" thick. "Big" for this car at least. This was around 140,000 miles.

Voila no more issue. Unless I tried hard to provoke it in hot weather. Now it has 210,000 miles on it and I've only seen the slightest of this surging maybe five times since then, and only when provoked. Otherwise it shifts as well as when I bought it 130,000ish miles ago.

Did the auxillary cooler magically fix something? I don't think so. Maybe lower temperatures made the TCC solenoid slightly less sad? Maybe cooler TCC clutch grips a little better? Maybe cooler Dexron is thicker Dexron and doesn't seep past something quite as much?

I don't know.

Just thought I'd share my experience. Hopefully it helps someone out.

Questions etc. I'll try to keep an eye on the thread.