350 motor keeps going out of time

[C101971]

So this has been an on and off problem for me, not frequently but not rarely. I time my engine to 8° and my truck will run fine. 71 GMC c15. And out of random sometimes it acts like the engine is going out of time or like a lack of fuel. It was so bad today I pulled over right away cause it seemed worse than usual. I check the carb, fuel is flowing as it should through the Jets. I check the filter, it's not clogged. I check the fuel sending unit, no blocks. Check all vaccume lines. They are all sealed. Spark plugs and wires are new. Spark plugs we're fine when I started the truck but half way home it started to act up.

I have not checked the timing yet as I just got home but usually the timing is out when this happens. My belt is only driven by the crank, timing and alternator and the belt seems tight. Alternator is mounted as far as possible to offer the most tension. It is a Very belt so I know they can slip when put under heavy acceleration.

Any clue as to solve this problem? I am running out of solutions