Truck won't turn over

I have a '00 1500 Z71 with a 5.3L. Last night after driving to a retaurant I had trouble starting the vehicle. After several attempts at turning the key and having nothing happen it finally started. This morning I went out started the vehicle without any problem and drove to the store, when I came out of the store it wouldn't start at all, again I had the same problem I would turn the ignition and all the lights would come on but the engine would not crank over.
I had this same problem back in November and I replaced both the battery and the starter and that seemed to have have fixed the problem until now. I thought it might be the starter again so I took it down to the local AZ and it tested fine. I've hooked a voltmeter up to the smaller wire on the starter and had someone crank it and I was getting power.

At this point I am out of ideas and would appreciate any help. On a side note I'm currently unemployed so taking the vehicle anywhere where they might charge me more than $100 is just not an option.

Thanks

when the vehicle fails to start move the wire lug at the starter...if the starter then cranks retorque all fasteners especially the electrical fastener that connects the solenoid to the starter motor ....

I have found this fastener to be loose on many starters lately..

remove the battery ground first before securing these wires...