Sorry, I did not understand. You want to randomly move the bomb every X seconds, or spawn random floor parts ?
If it's moving the bomb to a random position, it's something like that :
Every X seconds => Set bomb position to X(random(x1, x2) and Y(random(y1, y2) with x1, 2 and y1, 2, being the intervals.
If you provide a small capx, we should show you more precisely.