Anyway, I drove it today with the A/C on for about 15km and everything works fine. Shut the car and went to transfer my rego (approx 30 mins), went back up to start the car again with A/C on, driving down a bit (20-30 km/h) and when I depressed the clutch, the rpm drops down to about 100-200 rpm. And then I tried it again, rolling at 2nd gear and depressed the clutch, and this time it stalls the car entirely (happened a few times). With the A/C off, repeating the same thing, the rev dips down to about 700-800 rpm.
The other thing is when is idling, and then switch on the A/C, there's some sort of rattling noise but goes away when you depress the clutch

Details of the car
1989 MX5 NA6 (t28 turbocharged)
Haltec PS1000 ECU (Installed last thursday and tuned this tuesday, lots of delays from the workshop)
Just did 160,000km service
There's a 5th injector under the intake piping (right before the throttle body), planning to remove it
P.S I found this while I was searching http://www.miata.net/garage/KnowYourCar/S7_Idle.html but I don't think is the same thing though