106 diesel engine revs when clutch depressed

this is an intermittant fault occurring mainly between 3/4 gears and so far has only happened when I change up through the gears. i had the
clutch assembly replaced in january this year. it sounds like clutch slip but it occurs when i start to depress the clutch pedal in preparation to change gear. am wondering if there's a chance the accelerator cable is sticking, or maybe a leaky hose somewhere? there are no obvious leaks or drips under the car. Any ideas?! cheers!
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