1989 gti 1.6 loosing power when engine warms up ok when cold

First thing i would check would be the fuel mixture. you will need a gas analyser to check the CO% from cold start through to 'normal' engine running temperature. Obviously the mixture from cold start will be or should be 'rich' and as the engine warms up the mixture should weaken off. Bearing in mind the age and mileage of the engine and related fuel system components the manufaturers spec is not necessarily acheivable but the mixture should be around 2.5% - 3.0% at idle at normal running temperature. Other issues with these old classics were breather hoses blocking or split. check all external breather system hoses if they feel like they are made out of chewing gum and are easily squeezed flat then i recommend you replace them.