2005-2010 Vibes have a drive-by-wire system, where there is no throttle cable, and the gas pedal is just an electronic switch that sends a signal to the engine. I suppose it could be failing, or there could be a loose wire or connector somewhere. Or it could be something else completely. Did this car have the ECU recall done?
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Note: There was a huge kafuffle about the pedal years ago called "unintended acceleration" where some accused Toyota of covering up defects with these pedals, however in the end nothing was proven. Later model cars got a recall for a retrofit that would kill the engine if the brake and gas were applied at the same time, but this only applied to the 2009 and 2010 model years.