Could be a bad pump.
However, I would tend to think you may have a lot of crud in the system. You changed the filter but did you clean the lines at all, flush the tank, check the air filter, at least clean the plugs, etc.
If it wants to die under load, i.e. throttle use, it could be one of those things restricting firing, gas or air flow. I helps to clean out the whole system so that any crud is gone.