Hey everyone, first time posting

I have a 1981 datsun 810 diesel and Im having problems with the electrical system. It takes a bit to get it started in cold weather, and it has to turn over quite a bit before it gets going. Recently it starts to turn over and then I lose power and it turns over slower and slower. I just replaced the battery and alternator and it still is having problems. I don't think its the starter either because when I first turn the key, it turns over no problem... its only after a couple of seconds that it starts to slow down.

Does anyone know what else it could be? Is there a voltage regulator in these cars? I would rather not take it to a mechanic because the car is pretty rare, and I dont think anyone around here would know. If anyone knows of a old school datsun mechanic near Salt lake, let me know or I can do it myself as long as its not to heavy duty. Thanks in advance!