Your 85 Mercedes 300dt has no power run good on a flat freeway until you hit a hill Seems like no turbo but you can hear it?

I have an 83 300D...same trouble..Hauls on flat lands...even makes hills if it's warmed up and good speed, but in town, can walk faster....I've been told I have vacuum leak. Chasing that now ...hope it works. I have a new Turbo so that's not the problem.