Novak HV 6.5-Issue help needed!

[jamiekerr14]

Hi guys Im running a
1/8th Brushless RC8 Conversion with the Novak conversion kit with;
Novak HV Pro 6.5 Motor and esc
1)Hyperion G3 5500 35C Brand New Lipo
1)Flightpower 5000 25C Lipo-Lighly used
SR1000 RX
And newish Savox-1258
And New EC5 Connectors

Im having a issue.. which I cant work out.. If I do loads of steering ect and sometimes after a while it will either slow and only turn half instead of full lock. Or will stay straight until I let off everything then it seems to be ok again.. for abit then will happen again. On the instructions it shows if this colour shows this is the issue but it flashes red and amber when it stops. Which isnt on the Problem solving. At 1st thought it was the servo so tried another and still same issue... And changed to a newer rx and still same. The batts are fully charged too.. what could be the issue. Its like it would happen on low battery but its on a full battery.. Like it cant handle the voltage.. the rc8 and motor esc was brought of a mate. Which its only been used few times.. So its not like its been killed before.. any help really appreciated..