Any help?

my nissan exalta GS 2002 have a problem. the prob is when it goes to Drive shift the RPM abruptly down to 650:

P:N = 850 RPM
D = sudden 650 RPM
engine start = up to 1100 RPM then back to 850
AC on = RPM does not change

i already send it to the mechanic and cleaned the throttle, overhauled the radiator, change spark plugs, but the problem still ex

any help please!!!