If you haven't seen my post regarding a bad coil that I thought was causing this problem--just know I had a bad coil on 2/5 (number 5 misfire) after doing some work on this car -98 3.8...Anyhow, it was showing signs of what I will call an "rpm spike" on the tach of about 250-300 rpms...going anywhere from 30-65 mph in overdrive. And yes it is slightly noticeable so it is not just the tach. Background on the car is I got it cheap with a bad vibration which was said to be the torque converter. Installed a used transmission, new motor mounts, trans mount, plugs, wires, pcv, belt, fuel filter-ya know full tune up. Trans shifts fine, car runs great, but has this new little problem. It is of course hard to trace because so much was done at once.

Here is what I have checked:

All coils are fine, swapped out with others and re-checked
Installed used coil base from another motor I have
New wireset, could not find any burned wires from the old set < 300 miles
Cannot find any vacuum leaks although the vacuum hose near where the rubber intake meets the aluminum intake needed some attention
Put in some good quality fuel system cleaner just for the hell of it
I don't think it a fuel problem because that was not touched sans a fuel filter.

I fear it may be a torque converter problem, like it is unlocking and should not be because it is worn out. But it sure seems like it is an ignition related problem.

NO codes at this time

If ya got anything I sure would love to hear.

 

check your wires on your brake pedal. They may have gotten hot at some point and melted together, these control your "lockup" and turn the cruise control off when you tap your brakes. Mine melted a good bit and had pretty much the same issue your having. I wouldnt say its ignition related, but it is electrical. You may have to follow the wires pretty far up so scoot that seat all the way back and be prepared for the blood to rush to your eyeballs..... *try to find a bright penlight or small flashlight of some sort. It will help a bunch. Let us know what you find out. May also try unplugging and plugging those wires back into the switch if you dont see any melted wires. If those arent the problem, maybe replace the brake pedal switches from a salvage yard unless you just dont mind buying new ones.


Also while you under there, check out the wires going to your ignition switch as they get pretty burnt as well and can in some situations cause those symptoms. I think that is originally what melted mine, I just cut and spliced the plugs from a 90 somethin astro van in the salvage yard. I think I only had to repin one of the wires. Colors dont all match but if you do it one at a time you'll be ok. Its a regular GM ignition switch plug, and I wasnt about to pay $120+ from the dealer for new pigtails/plugs.

Spend some time under your dash and check as many wires as you can.