425 cat

Hello I own 1997 Ford L 9000 with a 425 cat all mechanical my problem is the run really good no smoke but underpowered turbo boost used to be 25 and now it's down to 15 on flat highway goes about 80 miles an hour as soon I it a small hill it drops down to 50 I hear the turbo kick in I check for air leak no leak change all filter does somebody have subjection I appreciate it thank you