There are 4 likely causes of your problem:
1. A weak battery (charge it & check it per tip # 49))
2. Dirty grounds/connections (remove the cables & clean the connections)
3. Bad cables or incorrect sized cables (replace them - see tip # 41)
4. Bad ground between the starter & the block. (pull the starter away from the block & clean the mating area - see tip # 36)
You have addressed the first two likely causes.
Another cause to consider after you eliminate the four most likely is a bad starter as Dean suggested.