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Rory-
Not much to go on here; as your mech told you, a code would be helpful.
BUT...you did say one thing that would move me to investigate in a particular direction: "the tachometer will widely oscillate at low rpm and the car will shake terribly as if it is going to stall. it has stalled around 4 times. there is often a strange smell to the exhaust afterwards."
If what you describe as a "strange smell to the exhaust" could also be called an acrid smell, then I would suspect a possible head gasket problem. What can happen is that coolant will/can sometimes leak from the cooling circuts across the broken gasket and enter the cylinder directly. When this coolant is burned, it gives off a very nasty and unforgetable smell. If there's enough coolant leaking, it will make the car shudder as you describe. You might not trip a CEL since most of the codes that would be effected would have two-trip logic which you did not meet.
I would ask your mechanic or dealer to check the oil for coolant contaimination. If there's water leaking into your oil, you can sometimes see it as a milky-brown coating on the inside of the oil cap. But if there's any contamination (the oil test is the most sensitive), then you'll have to have the head gasket replaced. The test is relatively inexpensive (about $25) plus whatever your mechanic will add to mail it off.
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