Why does my BMW keep overheating or getting an air-lock it drives well after bleeding the system.. for a few days then she either overheats or get an air-lock………HELP

Blown headgasket.

Big
Mechanical
Worry

it is possible that the impella has fell of the water pump it is common

hard to tell exactly what the problem is but being a BMW a cracked head is high on the list. Blown head gasket is much lower

your head gasket has got a fault or water pump impellor has perished or cylinder head is cracked,all depends on what engine it is.