Hello,
if I may add to the numerous very good advice. I see two possible paths for the stated behaviour
1) battery not able to supply high current
2) your connections
for 1) advice is given, easiest just check with another battery you know is good (in my case that would be the one used daily in the car)
for 2) I had and sometimes still have that on my boat. Connections like the clamps, blades inside the connector onto the outboard and on the outboard the switching gear all are prone to corrosion. Even with cleaning it builds up again and again (I guess we use the equipment near water
With the high current when cranking matters get even worse, the cable itself and especially the aforementioned bad connections heat up, as a result even more heat and higher resistance.
I did have quite some sucess in connecting a 2nd battery in parallel to the first one, just in case you want to try out something else
In any case, please leave a feedback on what you found out.
cheers
braq