My tata safari 2.2 dicor misses a beat around 40 kmph.....AC gives condensed moisture....?

my tata safari 2.2 dicor engine misses a beat at around 40 kmph when cold, is it normal for diesel engines?
also, AC of the same car gives white fumes (condensed water) after running for more than an hour. mechanic says it is it?
does it cause any harm to engine?
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