I drive a 1981 vw rabbit truck (diesel). The truck has been running fine for 8 months.

However, when i took her out today, the engine was very, very weak.
I had trouble getting out of my steep driveway, which normally is no problem.

The truck still feels smooth, but when I reached the peak point of a gear today, it was noticeably weaker than the day before. I live in a hilly area, and normally I can get up hills no problem, using 2nd gear sometimes on the really steep parts.

Today I had to cruise very slowly in 1st gear to get up the hills. It was like riding a lawn mower.

Any ideas on the problem? I recently rebuilt this truck
(new (used) engine, radiator,etc.) so I would like to try and fix the problem myself.