Is it throwing a CEL? You need to get the DTC read out. I had/have a problem with my crank positon sensor that causes the car not to start. Sometimes it just seems to start at the end of the cranking and doesnt catch. It always throws a CEL when it does this or it doesnt start. P0335.
The crank position sensor is on the back of the engine by the bell housing, drivers side. You could try unplugging it and plugging it back in and see if it makes any difference. Mine seems to have an intermittant connection/contact problem still and occaisionally wont start unless I fiddle with the connector.

MikeR

p.s. make sure the engine is COLD.