Ok, so when I bought my Datsun, it had a really bad clutch in it. I never really got a chance to drive it until recently because I just finished installing the clutch. Now the problem I have is that when I first start driving the car, for the first 5-10 minutes it runs and accelerates fine, but after that, it just loses power all of the sudden and I slowly decelerate until I have to pull over and let it cool off. Then it runs fine for another 10 minutes, then does the same thing. I'm not losing power completely, the car stays running and when I push down the pedal, it wants to go faster but it doesn't... it just slows down to about 20mph and I cant get it to go faster.

Check your fuel pressure.

Yes check the fuel pressure when it fails, the pumps often get hot and stop working, also the igniter and distributor can fail in a similar fashion, fuel pressure would be the place to start.