Swift Kotiki losing power and missing like mad

My father has a swift Kontiki that for the last year we have had constant engine trouble. we think we have changed or checked just about everything but it still does it. This is what happens.
whilst driving it starts to miss and loses power. this gets worse very quickly till you have to stop altogether. Letting it tick over for a few seconds will clear the problem for a while. Maybe a mile or a meter. Theres no pattern. Hills are the worst. Sometimes just changing gear clears it for a while. Also it can be as soon as you start a journey or after 50 miles, again there is no pattern. There is sometimes back firing but again not always.
Has anyone got any thought as we and 2 mechanics are stumped!!
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