2000 SAAB 9-5 2000 SAAB 9 5 SE Weird problem

Hi, We have a 2000 Saab 9 5 SE purchased in 2002 with 28000 miles. It's been a great car until recently, the check engine light comes on intermittently but the car performs as normal. Sometimes if idling or even just stopped in traffic, the check engine light comes on and the shift indicator goes from D to R, car remains in drive though. However, when stepping on the gas it has no power, like starting off in 2nd or even 3rd gear. Moving the shifter does nothing except go to reverse all drive gears same issue with no power. I can reset it by turning the car off and restarting, (i'm wondering if it's runnng Window VISTA). What do you think is the problem? I'm afraid to go to the dealer as we spent over $2600 over the past year to fix other things including an A/C problem where they had to take the whole dash out to replace a.10cent plastic piece so the A/C would come out on the drivers side.