Sounds like it is either tail or nose heavy as aeajr said it needs the CG balanced...

I can tell you this with the stock setup I had two battery packs a 7 cell 2/3 and an 8 cell 2/3 and I couldn't get the Electra Jet to fly with the 7 cell it seemed like it was just simply not enough voltage and current to motor for flight... The 8 cell would fly it but it was hard to control. I started playing with the CG and after I found the correct balance I put the 7 cell back in and she flew with no problem at all pretty slow but she would fly.

So the CG is everything on these birds...

BTW the weight of my 8 cell 2/3 is 6.2 ozs so I really don't think your problem is a weight factor I think it's your CG...