Id like to post a problem that stumped many in hopes this doesnt happen to you.
Please comment anything else to help others. Im not a mechanic

Recently the engine light on my 2003 Cadillac STS and the trans shop said the car wasnt going into overdrive. (i dont remember the error code, it was the one for input shaft sensor.) Had the trans rebuilt.

After doing this the car would go into overdrive at about 40-45 mph and id get a pretty serious bucking, I never had this before and I assumed it was torque converter chuggle. (weak overdrive lockup)

I took it back to the shop and they checked all the vacume lines and anything they might have disturbed. No luck. I took it to a caddy dealer who thought it had the wrong torque converter or had leaking internal gaskets or flow plates that were incorrectly torqued (weak internal oil pressue preventing the tc from locking completely)
The shop that rebuild the trans pulled it back out opened it up and could find nothing wrong, put it back in. Sent to a caddy master mechanic who ran fuel injector cleaner through it. Made a little better but not much (a clue)
The shop sent to a dealer who checked the computer software to see if it had been updated. Getting frustrated with mounting rental car bills etc I asked what led them to recommend the next step which was to pull the valve cover and check for a flat cam lobe. He said it was on a troubleshooting flow chart. Seeing that no thought was going into the process i brought the car home. I really was impressed the shop was going to such lengths to live up to their guarantee and will recommend them highly..

I was willing to spend a lot of time figuring this out so heres the end of the story
I replaced the spark plugs and noticed #5 was slightly red and #7 was full of carbon and raw gas. I re-gapped #7 down from the .052 i set it to down to .40 to see if i could get it to fire. No luck. I also noticed the several rubber boots i pulled had black and white looking stuff on them. Swapped the rubber boot for plug 1 and 7. no luck drove a week and pulled #7, no change

I Bought a used coil pack from ebay for $80. Put on the Odd cylinder bank. Success! Drove car, big improvement but chuggle wasnt gone. Bought another coil pack for 90.00 that came with 4 used boots. Installed and swapped the strange looking boots out with the ones i bought. Even better but the chuggle was there 70% of the time. I pulled the plugs, #7 was firing. nearly clean.

At this point the car idled nearly perfectly, only a slight unevenness, To have it still chuggle was frustrating but at this point the chuggle was 1/2 as bad. Then i installed rebuilt fuel injectors for 120.00 . DONE~! No Chuggle.

I knew the car had a miss while idling but I thought it was intermittant and i was going to do the plugs soon anyway. I never realized how much the overdrive lockup magnifies any engine roughness there might be. My lesson is to not put off problems till later and when diagnosing engine roughnes, start by looking at the plugs..