Well check that coolant temp sensor...thats a good idea. When it fouls out the car will run too rich which can cause stuttering. Also misfires can cause that, and will make your wideband read rich. 10.5:1 isnt really that bad though. Alot of stock turbo cars run around 10:1.

I would, in this order:

check coolant temp sensor
do a boost leak test (looking at your couplers wont tell you shit...you need to actually pressurize the system and find all your leaks if you want the car to run good)
inspect the ignition system...pull spark plugs, check, regap if necessary, or try new plugs...test wires...check coil packs for melting, missing insulation, corrosion etc...

then get back to us if that doesnt solve anything.