Blue may be temp sensor is faulty .Telling computer its hot, and in doing so its weakning out the mixture.Making it run weak. Also it may be fuel related .So a fuel pressure test is needed .Low revs at the lights could be down to bad fuel filter .Have you dhecked the idle control valve .It sits under the filter box .Remove it to look .It as 2 hoses going off from it , these can split ,and a plug on the rear .Remove it from the engine and clean it out with white spirits.Dont sink it in the liquid as it holds electrical items.This can change the tick over if gummed up inside..But if your sure its vacuum then its teasting time ,dont forget the vacuum hose in the front lights.