Where's the 'diesel' climbing power?

Hi guys!

I had this rather embarrasing experience just the other day:

My friends and I decided to go to the movies in this new mall. The three of us (we're all light-built, you'll see the relevance of this info later) rode my old-reliable 1976 w123-200D. As with all malls here in our area, this one had a multi-level parking space. As I entered and had to stop to get the parking ticket, I was a bit surprised to see that the entrance had an unusually steep incline (though Im not sure how or what 'grade' it was). And so, I gunned the engine (my car has a 4-speed manual tranny) in preparation for the climb. As I released the clutch, the car moved forward but as I approaced the steep incline, the revs dropped and I had to step on the clutch and stop because the engine almost died. I double checked and made sure that I was on first gear. I asked the parking attendant to stop the cars at the back because I might have to back off. Still stationary on the inclined area, I again tried but the same thing happened. What I did then was to back off to level ground and then accelerated towards the incline. Again, the car failed. And so I tried a few more times and took a longer starting point and finally, I did it. The incline was just about 20 meters. Im sure that the problem is not on MY driving. What added insult to injury was that the woman behind my car in the small Toyota seemed to climb the same incline almost effortlessly. I could just read the thoughts of that parking attendant, and those of my friends, since I would always praise Mercedes, or in those instances my car in particular, for all its abilities. Aren't these models supposed to have good climbing power? My engine is ok, since we just rebuilt it about 5months ago. At that time, we also had all the fluids and filters replaced (though this car has the mesh-type air cleaner; just cleaned that thoroughly), and about 2 months ago I had the clutch-plate replaced as well. The car has been 'broken-in' and has been adjusted to specs since then. Well now, I have to ask... what's wrong with it? Is it fuel delivery? What I haven't done is to have the injection pump checked/replaced. But the car can still run above 100kph though being the normally-aspirated diesel powered car that it is, it takes a looooong time to get there. Neither does it have the overtaking power of gas-fed cars, but its acceptable nevertheless. It does emit the normal amount of smoke you would expect from a diesel car (nothing obstrusive even at high engine speed). Im not as happy with its mileage, though (6-9kms/liter stop-and-go traffic, 9-11kms/liter on the highway. Is this ok? I was told it should be higher since it only has a 2-liter diesel engine.). One last thing, I think we installed a 1979 w123-300D (non-turbo, auto trans) differential on this car (that's another long story but its done so...) which does not seem to have a marked effect anyways, until now. Im not sure but I think it has a different ratio. Forgive my verbosity but I just had to give the background. Please help me reclaim my bragging rights for this car . Thanks in advance!