once in a while 与occasional, at times, sometimes的区别

1.once in a while
有时
He went to see them once in a while.
他有时去探望他们。
2.occasional
偶尔的,非经常的
She likes an occasional glass of wine.
她喜欢偶尔喝杯酒。
He pays me occasional visits.
他偶尔有时,间或
3.at times,
不时
Everyone may make mistakes at times.
每个人不时都会犯错误。
4.sometimes
有时,间或
It is sometimes warm and sometimes cold.
天时热时冷。
She comes to visit us sometimes.
她有时来访问我们。

可见这4个词只有细微的差别，可通用

没有很大的区别...我感觉是这样一来:
once in a while 最小
at times
occasional
sometimes 多点
但基本上,他们没有什么区别.我分成这样字也只是我个人的感觉...别的外国人可能想的不一样.
hope that helps