Have you changed the intake Gasket you said in your first
post, that the mechanic told you was bad, back in January? The gunk on your radiator cap is the result of oil getting into the coolant system. Now your Coolant is discolored. You need to do the upper and lower intake gaskets. Have your Cooling system flushed and put a new Tstat in.
I hate to say it but you may be on the road to blowing your engine because if they is oil in your Coolant there could be coolant in your oil.

 

Yeah-I took it to the mechanic this morning, after having no luck getting it to run cooler yesterday. I feel like I'm in over my head on this one and the last thing I want to do is blow my engine. I'll let you know what they find but will have to get those gaskets replaced.

 

Just wanted to post an update-took them a while, but the mechanic got my car back to me Saturday. Apparently, it was not actually overheating. The gauge was showing that it was, but plugging directly into the computer, it is running at the spec temp. He had to rev it to 2000 for about 10 minutes while sitting still before he got the fans to come on at 221 degrees (which is the factory spec). He said they came on every time the moment they were supposed to, so there's nothing wrong there.

Mechanic also checked out the intake gasket. He said there is a very slow seep of oil coming out of it, but that it is not mixing with coolant. He advised not to worry about it. Apparently someone prior to myself replaced them with Problem Solvers-he said those are much better and that they should last a good long while. Overall, he said, my Monte is in great shape. Glad to know everything's working well. Thanks all for the advice!