ok just bought a bmw 1999 520i auto when i start the car and drive it it drive great but after around ten minuites when i need more power its not fully there if i use the kick down the power is not 100%

if i stop the car and start it again the full power is there and the kickdown is super as it should be would it be coil packs / spark plugs or something else before i start buying things for it