I had this problem and the guys here on RCU helped me solve the exact same problem. I had to check 2 things. The first was the incidence on the stabilizer. The second of course is the CG. The first thing I would do is check the incidence on the stab and then reduce the amount of weight in the tail. Double and triple check the CG as previous stated in the inverted position. Also make sure you have down thrust on the engine...this should already be factory set like the stab but make sure they are correct.

Just this week a boy and his father showed up at our field with a v-tail 3 channel park flyer that would not take off. The tail would lift but would not leave the ground. We hand launched it and it flew 30 feet straight to the ground. After close examination I could see that there were two thumbscrews holding the stab. I adjusted them...changing the incidence and the plane flew great the rest of the day. I say all this because I have run into this situation in 2 ARF's. One was similar to your Kaos (Goldberg Skylark) and th eother was an Albatross biplane.

Good luck I hope some of this helps...let us know.