Why does my truck would do a quick stop and jerk even if I did not hit the brake

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My truck, a 2006 Mitsubishi Raider Duracross would suddenly do a quick stop and jerk like somebody would step on the brake and then quickly step on the gas. It really is frightening bec. this usually happens when I'm going on high speed. The problem started two weeks after I bought it from Budget Motors in Reno. Today we drove it to Mt Rose and we experienced the same scary problem several times while going up to the higher elevation at a speed of 50-60 miles, then the engine failure symbol started to light up on the dashboard, but the truck continued to run, thankfully.