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Your first question: strange, never happened to me.
Second one: probably your altimeter setting is badly wrong, change it to the current one or just 29.92inHg or 1013mbar, that should give you more accurate readout. If you use old style gauges, just turn the Kollman's knob on you altimeter, or if you use G1000, here's the manual, page 19.
Hope that helps.
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