I have a '77 Monaco Brougham with a 318 that is having some odd idle issues.

It will idle normally from start-up for about a mile or so, and then will start to idle high -- and will go higher and higher until I tap the throttle. It will be fine for a few minutes and then idle high again. The car doesn't do it every time I drive it, but it has been happening more and more.

I've checked the return spring, cleaned the carb/throttle plate and it has new plug, wires, 120k miles. Any help would be greatly appreciated!!!!

 

If it has Lean Burn it could be the Throttle Position Transducer. There are other inputs to the Spark Control Computer which could be advancing the timing raising the engine speed at the wrong time.

Does it have this system.

 

If it has the lean burn system this is a good time to trash it..
Could also be worn out throttle shafts

 

I do not believe it is a lean burn car, but this is the oldest Mopar I have owned and am a novice with carb'd cars. How would you know if it is a lean burn car? I mean, I don't see a spark control computer or any other oddities...

Worn out throttle shafts? This is a new one to me! How would one go about checking if that was the issue? --- thanks for the input guys!

 

I believe there is a stepper motor on the throttle that if the contact on the end gets dirty/corrodide it will cause the issue you are having just wire brush the contact or sand it with fine sand paper. And see what happens.

 

Lean burn system has a big box hanging off the air cleaner. The throttle shafts can be checked by moving the shaft up and down checking for play and to see if the idle changes. Make sure the throdle cable is not too tight

 

Sounds like your fast idle is getting stuck somehow. It is a plastic step looking thing and there is also a setscrew to adjust it.