Need help on 380 SL

my 84 380 SL bogs down when I put the kickdown in to use. Other wise the car runs fine. I replaced the fule pump relay to get the car started again but
have noticed whenever I put the metal to the floor the car makes a strange sound and starts to bog., if I let off the car runs fine. It goes into normal passing gear but bogs whenever the kickdown is called for. Any answers???
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