Lexus Repair: Eratic Performance, batery, performance problem

Question
I have an engine performance problem.  The check engine and trac off lights come on periodically and the engine sputters and sometimes stalls, but starts up without delay.  It started doing it on occasion, then once a day and has progressed to the point that it does it at  startup, but not always.  When both check engine and trac off are lit it will barely accelerate and the A/C doesn't work, but they don't always remain lit for any specific amount of time.  It does this whether the engine is cold or hot, but doesn't register any code in the computer, I've had the computer check 3 times.  When the problem occurs, the voltage drops to around 11 to 12 volts.  The alternator and the batery have been load tested and appear fine

Answer
Hello, I guess my first question would be who has been checking for the engine codes? The dealer? If the check engine lamp comes on there should be a code unless the computer is having a problem or there is a power loss to the computer which will clear it. Getting the code is the first step. Which Lexus do you have?