Make sure you don't this wiring problem on the + battery terminal. Mine was doing the same thing with the lights turned on, but would crank up right away. This told me the battery had enought kick to turn the starter but not the lights? It did not make since that is was the battery. Here is a pic of my problem - a high resistance wire connection. I just cut off the connecter and put a clamp on one. Corrosion of the wire here will cause that problem. Also check the status of the alternator blue ground wire and the other ground wires.

http://mywebpages.comcast.net/wduke6/volvo1.JPG

(case sensive)