If it is automatic you have problems. If it is standard you still have problems. Put some Transmission Fluid in it then if it still "slams" you need a new transmission.
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The transmission is not slamming itself yet you are experiencing the gear drop in rather hard inside the casing. Check your transmission fluid level and make sure its full. Also check the consistency and color of the fluid to see if it needs to be changed (filter also will need to changed).
I had an old Pontiac do this very thing and it was because it was extremely low on trans fluid. - jedgell233
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