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It sounds like the IAC is not working properly. The ECM uses it to allow more air in to maintail the RPM as load increases, such as pulling into gear or turning on accessories. The motor only makes about 5hp at idle in neutral with no accessories running and that alone isn't enough to handle extra loads.
Remove the IAC motor, clean the pintel & bore under the TB as well as the TB plates. Then reassemble and use the set screw on the side of the TB to set base idle (with pintel fully extended) to 450 RPM. If that doesn't do the job, replace the IAM motor.
BTW, my L98, when warmed up, also idles at about 800rpm in neutral and in gear at about 600rpm or so.