1995 Mercury Tracer

My daughters car runs great for the most part. It has had new brakes, tune-up, fuel filter, cat. Converter, plugs and wires, many other things done to it but it has decided it doesn't want to go. If you are sitting at a stop and push on the gas it moves VERY slow for about a blocks distance and then its fine. It runs fine once its going but if you slow down or stop its the repeat and doesn't want to go. What could it possibly be, im tired of changing out things that it could be. The diagnostic said nothings wrong, no check engine light but its ready to be ditched. Help me please.