Have you replaced your gearbox mount under the battery box?
An old/worn one of these can generate a lot of judder during "Takeoff".

Also check the gap/gaps either side of your top engine mount. the two pyramid rubbers points should be touching the metal bodywork/cradle either side.

an air gap can form over time as the rubber shrinks.....also the metal cradle will widen over time. this gap will allow the engine to jump/judder back and forth.

this can be rectified with a wooden stick/pole as a punch and a hefty hammer, hit the metal cradle until there is no air gap between the rubber pyramids and the metal cradle.

you can also fill the air gap with something like lino sheet if you dont feel like getting all caveman on your 306.

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