A very common problem with vacuum cleaners is a broken conductor inside the cable - at the plug end - caused by over enthusiastic tugging over time. I have fixed perhaps 10 cleaners by simply cutting away the plug and a few inches of cable and by fitting a new plug. Quick cheap and simple. Even if it doesn't cure your problem the effort is minimal.
Another issue may be the over-heat cut out operating when it should not - these things test the temperature of the motor and will be fixed to it - you may have a temperamental one.
Other than that - as you say - the internal filter which you've cleaned.
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