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Erm, it's been a few years since I owned a 25 (still my favorite car). I never had that problem with mine, but did have similar with BMW 316...
Don't know if it will apply, but the BMW had an idle control valve (a sort of pistony looking thing) mounted on the side of the carb. When hot it would play up, opening the butterfly valve and closing it again in a bizarre attempt to idle efficiently and smoothly, resulting in high revs when hot, or when in stop-start traffic.
I was advised to replace this valve and the control chip by the garage, but a quick squirt of WD40 seemed to do the trick.
Can't find my old Renault manual to find if 25RTS has the same system, but it may be worth a look...