Power loss, slow response when eccelerating, lose power and speed on big hills. on 2002 Volkswagen Golf TDI

In the warmer weather it seems to run ok but not in the cold weather. My mileage has also dropped to 25 to 32 mpg, 32+/- warm weather, 25+/- cold weather. It's also slow to start when the temp drops below 40 deg.

Asked by for the 2002 Volkswagen Golf TDI