MBX6T Steering problems

[SV6000]

Hi guys I posted This in another part of the forum but I thought I might get more help here. I was having a few steering problems with my 6T not having enough steering so I put in a hitec HS-7954TH and changed the internal bec on the mamba XL to put out 7.4v. Unfortunately I am having the same problem. When the car is stationary the steering is really good but when I am running the car at any sort of speed I loose a lot of steering. It's as if the servo no longer has enough power to turn the steering.
the car is running a 1521 1Y with a mamba XL on 6s

Any ideas guys