Start by cleaning battery connections and inspecting the cables and the connections at the starter. If those check out it's pretty likely that your starter is going bad. Tapping on it before trying to start will sometimes get it to work a few more times before it give up altogether.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.