Is there a name for this?

[Peonyblush]

Not depression. Not anxiety. Person in question functions extremely well on a day to day basis; genuinely quite a cheerful, upbeat and sociable person with above average IQ and able to hold down a high pressured job, relationships and friendships without any trouble. However when something "bad" happens it plunges them into what can only be described as a black pit of despair. Life suddenly becomes overwhelming, they cannot possibly imagine anything ever being "right" again and the problem seems too enormous to solve. Everything is directed internally and they struggle to verbalise how they feel. It feels almost rage-like but the rage is never directed at anyone else, only internalised. Could be triggered by an argument with a loved one or a catastrophe at work.

A few hours later the despair has passed and they feel almost embarrassed at their reaction, but it is something they have no control over in the moment.

Is there a name for this?

It would be "affective instability" I think

Catastrophising? I think it's generally part of an anxiety disorder

Borderline Personality Disorder.

I'm not sure what the name is, but it seems like this person can't cope with the dualistic and uncertain or unpredictable nature of life. That's likely to be a result of how they learned to function in the face of adverse or critical events in their childhood years. Everything felt 'right' when they were given positive messages about themselves, they felt comfortable identifying themselves with the 'good', and felt in control when life felt 'good' or non-threatening. But they probably weren't approved of for being or feeling 'bad' at times, and it's likely that when things went 'wrong' in family and/or school life, there was a lack of adults modelling coping behaviours, stable responses and calm approaches to problems or challenges. As an adult, a lack of control over things going and staying 'well' or manageable feels like threat and chaos, and they default to an overwhelming frustration and intolerance for the uneven, 'not-nice' aspects of life and relationships. Chances are this person blames themself for things not always being 'good' or 'perfect', as though they ought to keep things consistently 'up' and 'copable with', which is very unrealistic. He or she isn't comfortable with discomfort, and it sounds like the often imperfect nature of life and people is experienced as unacceptable, a feeling which is fearful and probably carries shades of guilt, shame and feelings of rejection. Those difficult and anxious feelings - internalised - then form a state of mind and feeling which is paralysing. I wonder if there's an over-inflated sense of responsibility.

I'm no expert, and what I've written above is only a partly-educated guess. But I sense a psychiatrist might diagnose a mood disorder or a form of OCD. No offence to the previous poster, but based on what the OP has described, I wouldn't jump to diagnose a borderline perosnality disorder.

I wouldn't jumpy to any diagnosis to be honest. If this person is finding their reactions to life events difficult to control than they could look at some online CBT or emotional coping skills stuff. Alternatively they might want to try groups if they are into that sort of thing, MIND can be good for this and has lots of local centres you can self refer to. If during these times they are a potential risk to themselves because of how extreme their moods are than I would suggest they have a chat with their GP pronto.

Could be bog standard stress or temporary anxiety.

Gosh avuncularis I think you've just described me.