The Savox servo/Spektrum radio combination is notorious for being unpredictable but in most cases installing a power cap in the receiver solves this, if you're sure all your transmitter settings are correct and you've double-checked everything else it may be all you need:
http://www.modelsport.co.uk/etronix-...roducts/368696
TBH it sounds like it could equally be something different but for £4 it's worth a try.