Check it out!!
Have you tried hooking a vacuum gauge up to it,to see if you have low vacuum??--It could still be a vacuum leak,as the other poster said,the carb base gaskets often crack or burn out,that will cause hestation(and should kill your idle too!)...there are many other things besides lack of fuel that can do what your describing--
a bad pick up coil in the distributor can do it,so can a bad accelerator pump in the carb--to me it sounds electrical,if it will run good sometimes and not good at other times--a fuel or carb problem would likely be there constantly---your EGR valve can make it bog out if its openning at the wrong time,or sticking open slightly--even an ignition module can do it too,it has a dwell advance curcuit in it,if that part of it craps out it can still run,but poorly-
-I use GM modules only,many "cheapies" dont even have that curcuit in them at all!--the pick up coil isnt much fun to change,you have to pull the distributor out and take the shaft out--so many of them stay in the vehicle till death do us part--and they can drive you bonkers with their erratic symptoms--I have seen some run good for days,then act up every day for a week!--the wires on them get twisted every time the vacuum advance moves the pick up coil to advance the timing--after many times the wires break inside the insulation,making poor contact at times,its hard to diagnose a bad pick up coil--replacing it is not expensive,and even if it wasnt the problem,if its got a hundred thousand miles on it,its best to scrap it,it will give you greif sooner or later..
You might have less than 12 volts getting to the big pink wire on the distributor too,if you dont have full voltage there it will have weak spark that will snuff out upon acceleration or heavy load,like pulling up a hill...there are lots of things to check--look for a gas filter in the carb that hasnt been changed,or any "universal" ones in the line hidden away under the truck that might be plugged---a kinked or collapsed fuel line can cause fuel starvation at times intermittantly too....Keep looking,and good luck!!
My 305 is running like crap again. Ok heres the deal. About 2 weeks ago I wanted to conquer my engine running crappy problems, so I Checked the timing (good), Changed all ......spark plugs, New distributor cap/coil/rotor(was old and crappy), Checked firing (good), checked compression (good), Fixed a vacumm leak I found (T-fitting broke) ETC... Well after I did that I was stoked because the engine seemed to be running pretty decent. Well that was until today. I started it up and it sounded fine, but as soon as I took off and tried to give it gas, it would completely "bog"/ "lag" out, NO power whatsoever.I even try to give it full throttle, and NOTHING happens... So I drive it down the street (not able to go over about 15 mph) and pull over. I shut the engine off, and check everything out, (everything seems normal too me). SO I start it back up, and still doing it just as bad. So I give it full throttle again to see what happens (
) And it lags/ "bogs out" for about 2 seconds and then all the sudden kicks in and takes off.... WHAT THE HELL IS GOING ON?!!??!
I am trying to get this truck running decent so I can start lifting/modding the thing out, but I want the motor to run good before I dump all that money. Someone please help, I dont know what else to do 
. I need a good hint on what to do, or even better, I need an expert to come check it out! Thanks. 
