All of a sudden my truck started this violent bucking motion. It's as if I were stomping on the brake, then the accelerator. It smooths out once I get up to speed, but the bucking is so hard the wheels squeal. I've got the three speed with overdrive standard transmission and a 300 slant six. Hardly ever had a problem for many years ...

 

It sounds like a clutch problem to me. Are the engine RPMs staying consistent or is it fluctuating? Also, how many miles are on it?

 

It doesn't seem like a clutch problem to me. No the engine RPMs are not staying consistent. The engine will die if I take my foot off the gas. It has 166,000 miles on it.

 

sounds like its not getting fuel, check your filter or make sure you haven't clogged your lines... Does this happens with low fuel on a turn?

 

No, it happens with plenty of fuel all the time.

 

Id start with the fuel filter and the spark plugs. Im assuming its a carb at this point... No fuel injection upgrades correct?

 

Look for vacuum leaks with all rubber hoses. The original hard plastic lines crack, so should be replaced with rubber.

Spray carburetor cleaner along the intake manifold to engine interface, while the engine is idling. If the idle speed changes, you have found a vacuum leak. Be very careful that you do not get injured by moving parts!!!

Check the base of the carburetor. There are two bolts with 1/2" nuts, which tend to loosen due to vibration. If loose, the base gaskets [carburetor and EGR adapter] will create a vacuum leak.

 

I'd consider taking a look at your distrutor too. I just recently had an issue like that with my truck (inline 6 - 300). The only difference was it drove perfectly until it hit 2000 rpm or higher. Then it started acting like I was revving and braking.

Turned out that the distributor had become loose. Retightened it (with some thread locker this time) and retimed. Runs like a top now.