I'm having the same problem. I'm using the 2 Amp charger that came with my HP Touchpad, which I think is pretty high quality and charges my Touchpad very fast, and the Spectrum actually loses power while the screen is on and I'm using it.
You can check your power draw by using Battery Monitor Widget. With the screen off the phone charges at 450 mA (at *exactly* that number, which implies the phone is limiting current), and with the screen on it drains at around -40mA to -50mA. There seems to be a software bug that causes it to draw less power while the screen is on. Of course it might have been a "feature" to reduce phone temperature.
Coming from a Droid Charge, 450 mA isn't terrible, although the Charge could charge at higher rates. I know my mom's iPhone can charge up to 700 or 800 mA. But even 450 mA should be enough to charge the phone while it's in use.