my Lexus GS 350, is it normal for the heated seats to shut off by themselves?

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When I put my heated seat on ( it has 3 settings) with in a few minutes or less than a half mile it drops to setting 2 or medium, then the same thing to 1 or low. Then it shuts off completely. This is all in less than 5 minutes . I have a 2004 Pontiac with heated seats, I set it high or low & it stays on, whatever I set it at. No problem whatsoever. This can't be normal to have to keep putting back on over & over again. Need help please !!