07m3

Hi I have a 2007 Mazda 3s Hatchback and am having a few issues that I'm hoping to get resolved with some help on here. The first issue is rough idle and drop in rpm's. It'll drop from 750 to about 500 and shudder the entire car. I am pretty sure it is because of the notorious bad passenger side engine mount but I'm wondering if I should go ahead and replace all three mounts. Do the rear engine mount and transmission mount go bad and relate to this shuddering issue as well?
I found an entire set on ebay. Thoughts on this?
http://www.ebay.com/itm/Mazda-3-2-3L-Engine-Motor-Trans-Mount-Set-3-PCS-4403B-4404B-4405B-M570-/350978140403?pt=Motors_Car_Truck_Parts_Accessories&hash=item51b7ed68f3&vxp=mtr
Second issue is that the AT light came up on the dash. This is for "automatic transaxle" so something is going on… I read the codes and cleared it but it popped back up within 300 miles. This is a 5 speed auto. The only thing I notice the transmission doing so far is revving the motor up to about 4000 rpm before it'll shift. Doesn't do this in any other gear. Also manual mode won't work with the light on. So is there a bad sensor that could cause this or is it a clutch?

I'd really appreciate input and advice because I really want to get this resolved quick.

icspots

If you're having idle issues make sure the wiring harness to your MAF sensor is intact, especially the ground. If you have an aftermarket intake they can stretch the harness and make it prone to breaking. Also if you have a broken mount it could add enough movement to stress the wires. Idle the engine and move that harness around to see if it duplicates the issues.

If you have a CEL then look up what the actual code is, i.e. the number it gave you. That's usually a great place to start with what the problem is. Note that you have an automatic transmission with a torque convertor. Yes it has a "manual" mode, but this is simply you being able to tell the transmission when to shift on command, and is not a clutch.