I bought this weed trimmer from Home Depot a few days ago. I put it all together (including an aftermarket spool that allows for fixed trimmer string...ie not a bump spool). I started it up and after it was warmed up, and I'd been using it for a little bit, it began to act up. If I'd pull the trigger it begins to sputter like it will die. However, if I gently squeeze/nurse the trigger (pulsing the motor to get the RPMs up slightly) and then go full throttle, it works fine. What could be causing this?

The only things I could guess would be bad fuel or too much weight from the new spool unit. Both seem unlikely. Ideas?