Sounds like a bad connection.
Check ALL of the connections in your battery cables, make sure they are clean and tight:
1. Terminals,
2. Lugs, isolator lugs, starter solenoid connection as it is commonly used as a junction post.
3. Earth cable / lug on engine block.
4. Condition of your cable at all of these areas (does it have corrosion in the copper?)
5. Follow all battery cables and check no one has put a joiner in previously. (they tend to attract corrosion)
Remember to check any earth connections as well as they are as important as the positive cable.
You could also check to see if the cable or connections are getting hot when you attempt to start it. (would point to the fault).
Hope this helps. Good Luck.