Did you put new points and condenser in the distributor? Bad points will cause your light to be erratic.
Sounds like your mechanical advance may not be working inside the distributor. This will cause the engine to hesitate when throttling up.
Check your timing at an idle not 2500rpms. Once you get the timing set then rev it up slowly and see if the timing advances using your timing light. If it doesn't advance then I would have a look inside the distributor to see if the mechanical advance weights are froze up. Common problem on the older tractors. Cars too!