I would hyphenate - the phrase 'sometimes angry' is acting like a single adjective, so to read the sentence fluently I'd make it look like one: 'the sometimes-angry tiger'. I sub-edit for a magazine and this is the rule we follow. To me, 'The striker is playing singularly' would mean that he is playing very strangely or very well.
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deborah_dI suppose it is acting adverbially as it's modifying the verb, i.e. the manner of his being angry is 'sometimes'.
deborah_dWell I always assumed that adverbs (which surely typically modify verbs) can modify some adjectives (or indeed other adverbs) because there's a verb implied - in this case 'to be'. So the phrase 'sometimes hungry' is shorthand for 'sometimes being hungry'.Interestingly, in many languages there are no adjectives. Instead, the adjective-like words act