。
The impression she gives differs from one moment to the next.
Literal
She [topic-は] occasion-by-occasion [in-で] receive impression [subject-が] differs.
A double-subject construction (彼女は ... 印象が違う) — 'as for her, the impression differs.' The relative clause 受ける印象 ('the impression [one] receives') has its agent omitted: who receives? The natural reading is the speaker, or 'one,' with 彼女 as the source. その時々で is a fixed adverbial: 時々 doubled means 'each occasion' (different from 時々 'sometimes'), and で here means 'depending on, at each.' The plain present 違う describes this as a general truth about her — perhaps suggesting changeability, mood, or a complex personality.