No power going uphill
 

I am very new to Volkswagens as well as this site. I have read some very valuable infomation here. I just purchased a 2000 Jetta TDI and it seems to lose power when going up hills. I checked the MAF sensor by unplugging it and it seems to be working. I changed the air and fuel filters and it performed better but still no power on hills. Any suggestions on what it could be or what I should do?

Thanks in advance!