In addition to misadjusted throttle linkage, other mechanical problems can cause an idle problem; for starters, low or no engine oil. I know it seems obvious, but it's true. If your engine doesn't have the proper lubrication, it will have to work harder to stay running, and as a result it might have an idle problem. So just for the heck of it, check your oil level if you have an idle problem.
Any extra resistance on the engine can affect the idle. Certain transmission problems can cause an idle issue, such as a malfunctioning torque converter. An overheating engine can affect the idle. If your engine is running hot, it can not only affect the physical movement of the engine, but also the air fuel mixture. An exhaust restriction can affect the idle. If your engine can't expel its exhaust, this can cause poor performance as well as idle problems. Here's a video that can help you determine if you have an exhaust restriction.
Another thing we can't leave out here are mechanical issues with the engine itself. If you have one or more cylinders with low compression, the engine will not be balanced as it operates. This imbalance can cause the engine to run rough or idle poorly. Also, a blown head gasket or burned valve can cause the same problem.
Incorrect mechanical timing can cause an idle problem. If you have an idle problem after changing a timing belt or timing chain, you might want to check to make sure you put the engine back in time correctly.
So don't rule out the basic components of the engine when looking for the cause of an idle problem. If the hard parts of the engine aren't working right, nothing works right. This is an often-overlooked item when diagnosing performance and idle issues. Use the Power Balance video above to find a cylinder that's down on power. In addition, do a compression test, or, better yet, a leak down test to determine the engine’s mechanical health. Here are some videos you might find useful for this.
Leak down testing
Don't rule out mechanical issues when looking for the cause of your idle problem. This is often overlooked, but should be part of any diagnosis.