Either the solenoid/starter brushes are sticking or the ignition switch is coming loose. Take the shroud off the steering column and look at the lower driver side of the column. There is a small screw holding the ignition switch to the column. Take that switch out and try using a flat tip screwdriver to start it. If it starts that way your net getting enough movement from the tumbler.
Id also start checking what voltage reading your getting to the solenoid. 
thanks that sound like it might work, It did it to me the other day real breifly then it started after I held the key for another second.
Thanks
Joe
