Boiling the wort

When boiling the wort is it better to keep the lid on the entire time? I had mine off and noticed my beer stopped boiling. After placing the lid back on It took about 5 min to get it boiling again. I'm thinking there was a 10-15 min period where it wasn't boiling. Is that a problem? Should I extend the boil 15 minutes?

Wort should be boiled uncovered - which allows dimethyl sulfide (DMS) to evaporate away. If you don't allow it to boil away, the DMS will give you an off flavor. If you can't get a boil going w/o the cover on, you need a better/bigger burner...

See: http://www.howtobrew.com/section4/chapter21-2.html

You should leave the lid off during the boil. While boiling, certain compounds are evaporating out of the wort. If you leave the lid on they will condensate on the lid and drip back into the wort. This is supposed to give your beer a sulfur-like off taste.
If you can't get the wort hot enough to boil, then you are trying to boil too much liquid. I'm assuming that you are doing it on your stove stop. Just add less water initially to the wort, and then top the wort off at the end with clean water. Toping off with cool water will also help you cool the wort down to pitching temperature faster.

I managed to put the pot on two burners and it boiled fine. Great advice guys'