It sounds like a bad connection, try cleaning the battery terminals and post. Also, check all the connections at the starter and starter solenoid. You said the battery is good therefore I cannot blame the alternator unless the alternator is not keeping the battery charged. Make sure all belts are tight and in good shape.
Good Luck
Dan
My goal is to save you money and time, I have a simple and logical approach for troubleshooting technical problems.
If you find my answer to be helpful, please remember to
ACCEPT the Answer!
Thank You!