Remove the battery connectors. Wire brush and clean the battery posts and connectors thoroughly and then firmly reconnect them.
99% of such problems are caused by bad battery connections. If cleaning your battery connections doesn't help, remove your battery and have a shop do a "load test" on it which will provide the battery's true condition.
NO, just looking at the battery connections and assuming they are ok doesn't count. My Toyota's battery connections looked just fine when I had the same condition recently but cleaning then and then securely tightening them back on to the posts completely cured the problem. Looking at the battery connections isn't good enough, they can look good but still cause problems.
And there's a 99.9999% chance this problem is not caused by your alternator or starter.