Erratic RPM and low gears shiftig late

Dear experts, here is my question. I recently bought a honda airwave 2007 model car which has cvt gearbox.
It has a problem unfortunately none of the mechanics who I went to was not able to figure out.

I start the car and start moving all seems ok but the low gears such as 1, 2 and 3 dont seem to shift smoothly it requires the rpm to reach approximtely 3000 to make a shift. Sometimes even to 4000 and sometimes "D" starts to blink so I have to switch off the engine and to switch off the blinking D

And when the car is get going the Rpm is not quite stable. It goes up and down constantly.

So here are thevthings I did to solve this issue.
I have replaced the gearbox to see if it solved the problem but since it didnt I got my old box back.
Done a tune up
changed EGR valve
Changed throttle body
Replaced plugs
Checked TPS
Checked cam and cranks sensors
Chaged cvt fluid

None of above solved my problem so here im here looking for help.
Any advices are much appreciated.
Thank you.