Hammo1

Hello guys new to the zf6 gearbox just have an enquiry just picked up an 06 xr6 bf with a zf6 and when I brake then go to take off if feels like a car has hit me from behind when it goes to change gear if that makes sence also this morning there was a bit of lag from when I put the accelerator down to when it actually decided to change gear could need a service just thought id get opnions thank you