If you have the Park Zone Decathlon I think you're analysis is correct and that you are stalling it. I have the plane and I love it, but you have to be a little careful. I stalled it the first time I flew it, but luckily I was able to save it. Here are a couple of tips.

1) Make sure you're at full throttle before you throw it (no kidding right?) and throw it *straight/level*. I'd say you want to throw it about as hard as you would throw a football to a middle school kid. Put a little effort into it, but don't rifle it.
2) Let the plane fly straight for 50 feet or so and build up speed. My plane will climb at full throttle with the nose just *slightly* pitched up. A good amount of airspeed will produce enough lift to climb.
3) Launch into the wind (if there is any).
4) Be very carefull of over controlling.
5) I haven't heard anyone with this problem, but my plane is very unresponsive in low rates. I keep it in high rates all the time.
6) Check your trim. Make sure everything is centered before takeoff (unless you've trimmed it out in flight before). Let's just say I may or may not have tried to fly with full right trim.
7) Consider doing some of the upgrades mentioned in other forums. I changed out the prop, 9x7 I think, but if you can't find any info, PM and I'll let you know for sure.

Hope something here helps.