2008 Suzuki Forenza car won't accelerate engine light rando

I bought a Suzuki in Set 2009, with 38000 miles on so out of warentee. My check engine light goes on randomly, sometimes the car drives fine, but other times I cannot accelerate and the car eventually recovers, other times it stalls out and will not start for about 5 minutes. Battery, lights, radio still work. Dealership could not replicate problem as when I took the car in, the check engine light was off and the could not duplicate the problem. It is a manuel transmission, and when car loses acceleration, down shifting does not work. Help!! I still owe $6000 on this car!!