zeitweilig
temporary
For a limited, temporary period of time; not intended to be permanent.
Wegen Bauarbeiten ist die Straße zeitweilig gesperrt.
Due to construction work, the road is temporarily closed.

temporary / occasional
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temporary
For a limited, temporary period of time; not intended to be permanent.
Wegen Bauarbeiten ist die Straße zeitweilig gesperrt.
Due to construction work, the road is temporarily closed.

The word is made up of 'Zeit' (time) and 'Weile' (while). Both parts refer to a span of time, which explains the meanings 'vorübergehend' (temporary) or 'von Zeit zu Zeit' (from time to time) well.
Die zeitweilige Schließung des Museums dauert eine Woche.
The temporary closure of the museum lasts one week.
'Zeitweilig' can be used as an adjective before a noun or as an adverb to describe a verb or situation. As an adverb, it usually comes directly after the verb or at the beginning of the sentence for emphasis.
Die Verbindung war zeitweilig unterbrochen.
The connection was temporarily interrupted.
The meaning depends heavily on the context. If it is about a planned, limited duration, such as a closure, 'vorübergehend' (temporary) is meant. If it is about irregular, recurring events, such as pain, it means 'gelegentlich' (occasional).
Für die Veranstaltung wurde die Straße zeitweilig für den Verkehr gesperrt.
For the event, the road was temporarily closed to traffic.
In the meaning 'vorübergehend' (temporary), 'zeitweilig' is often used in official or formal contexts. Typical collocations are 'zeitweilig gesperrt' (temporarily closed), 'zeitweilig nicht erreichbar' (temporarily unavailable), or a 'zeitweilige Einstellung' (temporary hiring) of staff.
Aufgrund technischer Probleme ist unser Service zeitweilig nicht verfügbar.
Due to technical problems, our service is temporarily unavailable.
sometimes
'manchmal' and 'zeitweilig' can both mean that something happens from time to time. 'manchmal' is the normal, simple form in everyday language. 'zeitweilig' sounds a bit more elevated or factual and is often used in written texts. In many sentences with the meaning 'ab und zu' (now and then), both can be understood, but 'manchmal' usually sounds more natural in conversation.

Im Sommer gibt es in der Wohnung zeitweilig Probleme mit der Hitze.
In summer, there are occasional problems with the heat in the apartment.
Am Wochenende liest die Fahrerin manchmal Zeitung beim Frühstück.
On weekends, the driver sometimes reads the newspaper at breakfast.