I come home at 4:00pm, and turn my AC which has been set on 85 degrees Auto to about 75 degrees. In the last few days, it has taken upwards to an hour and a half for the fan and AC to run. I have replaced the thermostat, and when I did and the AC wasn't working, the new thermostat seemed to solve the prob and the AC started up immediately. But, the next time that I turned the temp up so the AC wouldn't run, the system would not start up upon reducing the temp.

You compounded the problem by changing the thermostat before you new what the real problem was. Now you have introduced another possibility into the mix.

I would reinstall your original thermostat for starters and try to turn it on. If it does not run at that point then just turn the fan control of the thermostat to On instead of automatic and see if the fan runs. Then post back with the results.

Actually.. I have switched to the old thermo... it works for a while... overnight.. then, I leave for work, turn the system up... and when I get back home and try to crank the AC... it takes two hours for the system to start up.. I switched to the newer thermostat because that doesn't matter... and that's evident by the fact that the same prob is occurring with a new/old thermo..
Actually... as days go by... it takes longer and longer for the AC to start.Btw.. Thanks for the reply... I really don't have the $ to deal with a major AC problem or getting BS'd by a repair guy... so I want to bring some research/info to the table if/when I have to have a AC repair guy come out.. I work at a hospital and the electrician and another guy said they could come over Monday... The AC came on at 8:45pm tonight after being turned on at 4:45... so... at least I will sleep in a nice cool house tonight.. I hate to ask... but why does it seem that the system will run through the night... and if I don't mess with the dial? But when I turn it off... its hard for it to turn on from the off position?

Actually.. I have switched to the old thermo... it works for a while... overnight.. then, I leave for work, turn the system up... and when I get back home and try to crank the AC... it takes two hours for the system to start up.. I switched to the newer thermostat because that doesn't matter... and that's evident by the fact that the same prob is occurring with a new/old thermo..
Actually... as days go by... it takes longer and longer for the AC to start.Btw.. Thanks for the reply... I really don't have the $ to deal with a major AC problem or getting BS'd by a repair guy... so I want to bring some research/info to the table if/when I have to have a AC repair guy come out.. I work at a hospital and the electrician and another guy said they could come over Monday... The AC came on at 8:45pm tonight after being turned on at 4:45... so... at least I will sleep in a nice cool house tonight.. I hate to ask... but why does it seem that the system will run through the night... and if I don't mess with the dial? But when I turn it off... its hard for it to turn on from the off position?

Well, first you have to get the contactor in the AC to turn on.

Loose connections come to mind. A bad contactor is also a possibility. You know it isn't the thermostat.

It has been 100+ degrees here every day for the last many days... and I am wondering since the AC system is only running during the cool parts of the night/morning... is there any part of the system that may be in my attic or outside that expands and contracts and might be affected by normal wear and tear and is worsened by the extreme heat? It just seems strange that the system would work like a champ all through the night on auto... turning on and off as the house cools and heats up and cools again..

I am having a couple guys come look at the system tomorrow after work..

Guess I better get out my wallet :(

Thanks for all your help