The word you're looking for might be sporadic (sporadically) or random (randomly). ex: I go to the gym randomly.

I might add "inconsistently" as an option. Especially if you are referring to an obligation. "I promised my mother that I would go to the gym every second/other day, but I've been going inconsistently."

As a native speaker in the US, the words "occasionally", "sometimes", "not often", "rarely" seem most natural for what you want. For example, "I love cake, but I rarely eat it". "I occasionally go out and watch a movie. I would go more often, but it is so expensive." "Sometimes I put cream in my coffee, and sometimes I don't." "I don't study very often, but I always enjoy studying when I have the time." Note: rarely is less often than sometimes/not often/occasionally. So, "rarely" means "almost never". ------- As for the other words, I will try to help. "Chaotic" means completely confused and unorganized. "After the actor became famous, he became addicted to drugs, and his life was completely chaotic after that." It is similar to the word "haphazard". (Note: "chaos" is also used in an academic context called "Chaos Theory" which has a different meaning.) Erratic is also similar to chaotic and haphazard. It means random and unpredictable. For example, "Her mood is erratic. One moment she is happy, the next moment she is sad, and then angry, and then another moment passes, and she is laughing and smiling." Irregular means something is "not normal" or that it doesn't fit a pattern. Like irregular verbs in a language— they don't follow the pattern. The word "fitful" means something is interrupted by "fits". "I was trying to sleep, but the sleep was fitful" OR "I was trying to help, but the progress was fitful. People kept interrupting the progress." I should say "fitful" does seem to usually be used to talk about sleep. Also, the word fitful is rarely used. I am a native speaker and did not know what it meant.

I'm a "casual" learner and an "occasional" house guest, also, I'm an "infrequent" gym member. "fitful, erratic and chaotic" are more violent and edgy words. If I say "I lay down and doze casually" it means I drifted in and out of sleep in a pleasant and dreamy way. But if I say "I lay down and dozed fitfully" it describes something unpleasant. Like I was trying to get to sleep but I kept being startled awake with my bad dreams. "Erratic" has the same sort of edgy/bad feeling to it. "He drove erratically" means he drove a little crazy, or drunkenly, but "he drove casually" gives me a feeling of calm and stress free driving without rushing to a particular destination. Casual dining = means going out to eat without dressing up and being overly serious about it. Casual learning = means you're interested in learning but not in an overly structured or formal way. "Irregular" is a word that can have a bad meaning but it depends on the context. "The doctor says my mother has an irregular heartbeat" is a serious statement and "irregular" is the correct word here. You wouldn't say "a casual heartbeat" to describe the same thing because "casual" doesn't have the same serious tone to it. "Chaotic" also implies a serious situation. "The political situation is chaotic" sounds serious. "Traffic is chaotic" means it is crazy and stressful. But you could also say "my children are chaotic" to mean they are being noisy and running around. So "chaotic" is a flexible word.

'Occasionally' works fine, or 'from time to time', or 'every so often'.