alternator issue

i have a 01 Explorer Sport with a 4.0L SOHC and lately any time i use the power window at idle or the air, the rpm drops low and the car trys to die out. Now i am not sure if the alternator would be the problem, but when i put a mutimeter on the battery terminal(while the car was running) it was at 14.0 volts. i am confused what could be cause this. any help would be great.

Voltage is only part of the charging. You need to have the amperage tested also. Your accessories run on the amperage. So if amperage is low, charging system will not operate properly.

what amparage should i be looking for. 65a or 90a

That is dependant upon the size of alternator. The alternator should be charging about 3/4 of the amperes under load at idle. Normal amps at idle with no load will be somewhere atound 5 to 20 amps. At 2000 rpm, it should output almost 100% under load. Somewhere on alternator case, the amps will be stamped on it. You can have it tested at some auto parts houses. Just make sure they do an amps reading.

charging system was fine

How did you determine the charging system is fine? You can't fully trust the quick testers that the chain auto parts stores use. However, if they are confident that the charging system is working fine, then you have a weak battery or bad connection somewhere. A properly functioning charging system with good battery WILL NOT cause the idle speed to drop unless you are putting it under severe load and your Idle Air Control (IAC) is not functioning properly. Just using the power windows at idle though would not classify as a severe load. What does the battery voltage drop down to when you use the windows at idle? Is it a momentary drop, or a drop that stays low for the duration? A momentary drop would point more towards a battery issue. A drop that remains low would indicate more an alternator or connection issue.

-Rod