Why is my printer ML2851ND sometimes skipping top part of a page?

Unpredictably skipping a top part printing a single page, when happened needs to repeat 2-3 times before prints in full. Also printing in sequence on both sides skips some pages, mostly on the first side. And then not printing missing pages as singles (on both sides), but prints on one side any missing page, like keeping in memory the first refusal.