my 1982 380 SEC did that
is it drinking fuel? If thats what is doing check the air fuel mixture and adjust that.
if your car is a 1982 or 1983 you have two things controling you idle..
idle control valve..under the air cleaner its silver with a 2 prong plug on it that could be bad
idle control relay on the firewall a black looking box almost in the middle that could be bad also.
when I replaced my idle control valve my idle was way off and bouncy. Then I replaced the idle control relay it it stop the bounce in the idle at a stop sign.
the mechanic I took my car to get the timing chain done turned up my air fuel mixutre very high. to much fuel (will make it feel like it wants to go fwd when you have the brakes on) and I adjusted it and it doesnt drink fuel or want to go forward.
1982 380 SEC