Alcaline

Hello Scotty, I have a peugeot 307 1.4 16v petrol manual . What can cause the car to loose power going uphill. When I'm going Uphill on a monutain I literally need to downshift to 1st gear. When in I'm in 3rd oe 2nd gear it starts loosing power till i need to downshift and the hill is not that steep. On normal city driving it goes ok and on the highway it does ok too. A friend of mine says it could be the Map sensor. I cleaned it but I don't see any changes. Any ideas? Thanks