Help Needed

[350gt premium]

Hi all,

I have an auto 2004 series 2 350GT coupe 3.5 litre, I have recently changed the auto gearbox as my one was stuffed. Ever since changing the gearbox I now lack the power I use to have in the car. It's very sluggish off the mark especially going up hills it really struggles. I have taken it to various auto transmission shops, performance auto mechanics and they can't seem to find what the problem is, I have also taken it to Nissan however they refused to look at it as it is an import..... The diagnostic tool has been put on the car and no error codes came up. I'm stuck at the moment on what to do.

Can anyone offer any advice? Really don't know what to do.

Thanks.