90 Buick LeSabre shudder@35-40MPH

I have a 1990 Buick LeSabre with the famous 3.8 V-6 with 100K miles. I am having a problem with a shudder or surge at about 35-38 mph. It is noticable at all times going this speed, but mostly going up hill. I also here a noise immediately before it starts to shudder, it sounds like I am driving over a sewer grate and lasts for about a second. If I give it more gas while it is shuddering, it shifts up and the shudder stops. But the minute I slow down to 35-38 mph it returns. It doesn't appear to make a difference whether I am in Drive or Drive w/Overdrive. It seems more noticable after the car has reached normal operating temperature. I thought maybe it had something to do with the transmission, so I took it to a local tranny shop. They said they wouldn't look at it until I had an apparent misfire problem resolved. So, I have had it looked at by a couple of mechanics. Everything seems to check out fine, plugs, wires, injectors. One said that one of the coils was a little off. So he replaced the coil pack. $300 later, the problem still continues. Any advise? Any suggestions I can give to the mechanic on my next visit?

Just wondering, if your mechanics did a resistance test on the wires or, did they assume that since they looked good on the outside, they must be good on the inside? Your symptoms point to either “spark scatter” or a torque converter clutch malfunction. The 1-second “rattle” sound you here, could be either the transmission drive chain slack or backlash in the differential. This is not uncommon on a car with over 100K!:crying:

Ive noticed this in my 97 with 150k, is this a big issue that i should worry about? I havent gotten any check engine lights, so i d k.

My friends Bonneville was doing what sounds very similar. It ended up being the torque converter. They finally replaced it after the repair shop changed pretty much everything electrical to no avail...