why does the van keep shutting down when accelerator peddle is pushed.

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1995 G-30 HD High Cube Van. No illumination of Check Engine Light. Have replaced all emission parts including fuel pump, rebuilt throttle body, removed catalytic but still shuts down when accelerator peddle is depressed, will idle for hours and occasionally will not idle for 5 to 10 minutes. Problem is intermittent but more often then not, continues to shut down when accelerator peddle is depressed. At my wits end. HELP.