I think the trans fluid and motor flush idea is a good start. Did you change the oil when you replaced the VLAs? What weight oil are you using?

Water works really well to steam clean the carbon out of the combustion chambers - allow it to be sucked in slowly through a vacuum line or spray it directly into the inlet pipe. Far cheaper and more effective than adding anything to your fuel. If you feel that the water treatment isn't enough, pick up a can of GM Top Engine Cleaner - you'll feed that into the engine through a vacuum line and allow it to stall the engine as you reach the end of the can. After sitting for a while, restart the engine and revel in the mosquito-killing cloud of smoke it creates. Seafoam is supposed to work well in the same application but I have never used it that way.