Engine Started Over Heating

Hey guys! I currently have a 2000 mustang GT that has recently started to overheat. I have not done much diagnosing yet due to lack of time and money but figured I would see if I cant get an idea as to what exactly it could be or where to start. I first noticed the car running a little hot about a week ago. Normally the gauge reads just under the halfway mark. I was driving home from work on the highway when I noticed the gauge was reading just past the halfway mark just over a week ago. At this point the car would only overheat some times (usually if I drove for 30mins+) and was worse in city driving than highway. Fast forward to today and the car now overheats to about half an inch before the red line on the temp gauge after about 10-15 minutes of driving. I will also add that the car gets real sluggish once it is warmed up and remains that way the more it overheats. I haven't tried flooring the car because that would be stupid of me lol but I can tell it is bogged down just under normal acceleration and this happens the moment it gets up to normal operating temp and continues while it gradually begins to overheat. On my way to work today I decided to try and turn the vent on and turned the dial to hot with the blower on max and the car cooled off quite well. The temp lowered to just past the halfway mark and stayed there. I've tried searching this forum and google to see if I could find a post where someone had the same issue but have had no luck so I figured I would post here to see what could possibly be going wrong. I don't smell anything sweet from the exhaust that would indicate burning coolant and there is no smoke either. Thanks in advance for any help!

Edit: The radiator fan does come on without the AC on
 
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When engine is at operating temp feel the radiator hoses. Top hose should be really hot, bottom hose should be cooler. If this is not the case you could have a bad thermostat.
 
Your fan has a high/low setting. Low speed at about 2-3 minutes after start. At a certain temp (as when the AC is on) it will run at high speed. You need to check whether it is running at low and high speeds.
 
I had something like that similar happen to my Ford Mustang last year in the summer. What was causing my engine to get really hot and almost overheat before I turned it off was that my radiator was clogged up and my thermostat was not opening and closing correctly. I power flushed my whole cooling system and flushed it out really well with water. Then I replaced my radiator and thermostat and replaced the coolant. My Ford Mustang would never go past the half way mark on the temperature gauge again. Also check your water pump to make sure it is not leaking coolant. A faulty water pump is always possible.
 
Sorry for the late response guys and thanks for the help! (for some reason I didn't get an email notifying me of responses). I ended up replacing my thermostat since it was a cheap/easy install and it seemed to help increase the time in-between having to top off the radiator. I did notice a fair amount of dirt stuck to the sides of the expansion tank so I decided to have the system flushed and checked at a local shop. They didn't notice any leaks but agreed that a lot of crap was in the system. So far it has been about a week and the coolant levels are still full so I am suspecting it was a clogged cooling system causing it to slowly burn off coolant