The usual way of wiring lights is to snake the power cable from room to room which reduces the costs because less cable is used. It is not unusual to have one lighting circuit for upstairs and another for downstairs.
The power cable that snakes between light fittings carries Live and Neutral wires and an Earth conductor. If either the Live or Neutral wire becomes loose or detached at a junction point all the lights that come after this point will misbehave - flicker or just not work at all.
Your fault then may be at one of the non-working lights where an "in" connection is broken or even at one of the working lights where an "out" wire has come adrift. It could be either of Live or Neutral wires causing the problem.
You need to inspect (have an electrician inspect) the ceiling points for a bad connection.
Take care - good luck...
Peccavi
February 2009