1. When the car is idle there is a huge vibrate. But it is on/off basis. When I on the AC it gets doubled. But can't hear this vibration (noise) when the car is running.
IS this a carburator type or EFI cant remember on ASP model. anw i guess your engine is not stick on the correct idle level & may be a miss in efi or carburator or plugs etc. also check your AC IDLE up unit whether it's kick on after switch on the ac.check the engine mounts as JD net state.
2. Even the AC is off there is a light AC from AC vents. I have repaired my AC few months ago. Also the AC is very low cold during the day time.
may be you have a Gas leak. did you check all the lines & things related to a\c when refilling gas ?
3. When the car is switched off I can change gears normally. But when the car is running I'm getting a noise (bit tight) when changing from 4th to 3rd, 3rd to 2nd and from 2nd to 1st. But it is not happening when changing from 3rd to 4th and 4th to 5th also from 5th to 4th. One guys asked me to adjust the Clutch.
try to adjust your clutch first & see.