How old is battery? If about five years that may just be end of life.

ALSO check water level - they perform better when celltops covered.

Replace with another you have that you KNOW is good before you do anything else. If problems go away you know one of two things
1. Battery was problem
2 You cleaned the gunk off the connectors and now is good - for a while.

Put old one in for a day and confirm.

Dan Bentler