If liquid is sitting on a hot plate attached to a timer from *before Shabbos*, I don’t see why the hot plate going off and back on again would be an issue. Even if the hot plate is off when Shabbos starts, and one leaves raw food on it before Shabbos starts, the timer is already doing the melacha of meva’er by turning on the electricity. What’s the difference if it does the melacha of bishul too? There might be an issue of shehiyah, but that applies even on Erev Shabbos. (The heter, of course, is a blech.) However, Reb Moshe was against using timers on Shabbos in general, except for lights (and perhaps other things that are commonly used with timers during the week – some people do put their percolators on timers every night), kayadua.
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