They are almost the same. E.G. 1. 我偶尔会喝酒，一个月喝一次，但是不常喝。（"偶尔"更强调频率） 2. 我有时候会喝酒，节日的时候会喝，伤心的时候会喝。（“有时候”更强调我在某些时间或情境下会喝） 共同点：偶尔和有时候都表示sometimes，用在不经常的动作。

As far as i am concerned, I use 偶尔 when I tend to emphasize the frequency of doing this thing. When you are thinking about it and it occurred to you that you don't always do it and you want to emphasize that you don't do it very often, you will use it. Some times you hold this thing a bad thing and don't want the person to form a bad impression on you, you use （只是）偶尔 or 很少去, your tonque will reveal your true feeling about this thing. 有时候 is put in when you are reflecting the things happened at times. The things that follows有时候 is what you center on. The frequency is not your point. You just want to recollect this thing that happened some times in the past. Examples: 有时候他们会叫我去，但我通常会拒绝他们。/-你去酒吧吗？-偶尔。/我偶尔啊，只是偶尔会偷拿父母的钱去买东西。(you cannot say我很少会偷拿...in this case, if you use 很少 here, it gives me a feeling that you are totally okay about stealing from parents and you are a veteran on it. It's just not part of your life plan. But 偶尔 here gives you that you do do this bad thing and you are sorry about doing it but it did happen. It suggest that you don't want to admit it but it's high time that you confess it. It's tactful and roundabout that you use 偶尔 here) I think i overelaborated on it but i hope you can get it.

afternoon“有时候”强调的是自己主动的去做一件事，”偶尔“ 是被动的去做某事

They are interchangeable in most cases.

有时候>偶尔。