Sounds like the starter is going. Kerry's suggestion will tell you for sure. The only problem is that you'll have to pull the starter to perform the test, unless you're lucky enough to have one of those nifty current meters that clamp over the batter cable.

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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 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.