So when load is placed on the engine it bogs.
I think that probably rules out the catalytic converter and exhaust being clogged.
Could be a vacuum leak, when the engine bogs do you hear a whistle from the engine bay?
Could be a bad distributor, take it apart and inspect, if in doubt change it. Check your ignition timing as well.
Make sure your brakes disengage properly and that your handbrake is off although it would take awesome braking power to bog down the engine.
This is easy to test, will the vehicle move down a hill when in neutral?
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