an endless supply of bubbles can be a leaking head gasket.... compressed fuel/air from cylinder leaks into the water jacket during compression stroke.
the ECM is hiding on top of your passenger side wheel well under a black plastic cover, two metal buckles and the cover should come off. you'll also notice the very large black wire conduits leading to it. right in front of the overflow tank.
im clueless about the speedo. but i think if your cruise work, then its a problem wtih the instrument cluster or wiring to it. maybe if one of the other vets can chime in as to whether the transmission needs the VSS to control shifting, you could at least use that info to see if the VSS is the issue or something else.... ive never had to touch anything speedo or cruise control related on the lumina.