Idle drop when engaged to D or R

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Pls help:

Lancer 04 GLS, CVT

if I shift to R or D RPM goes idle down * around 750 to 600 and if I go back to N or P, RPM goes normal * around 1000 or 1100.

hindi naman namamatay yung engine but its kinda weird since if we dont need to press on the gas during traffic, nanginginig siya unlike before that it sticks to around 900 to 1100 rpm and runs smoothly.

and btw, while observing this, sobrang init and I think tuwing mainit lang nangyayari

Is it related to tranny kaya and do I need to replace my atf na kaya?
Or may kailangan lang linisin?

I cant do anything for now kasi wala naman bukas na shop.

I think my Cedia is starting to show its age na, going 7 years this may 28 and odo is only * 61,000.

Thanks

[incus]

change ATF if you haven't done it yet then observe..

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the last time I changed my atf was 2007 during my 40,000 km pms.
maybe I have to change then