Hello and welcome to JustAnswer!
I would verify the system charge level, but it sounds like you have a lack of airflow over the a/c condensor while you are stopped. While you are driving there is enough natural airflow over the condensor to keep it cool and keep high side pressure down, but when you stop there isn't enough natural airflow. The fan clutch has to engage to pull enough air through the condensor to cool it while you are at a stop. It sounds like your fan clutch isn't engaging to pull enough air through the condensor and will need to be replaced.
well, it is very intermitent - sometimes it works on the frwy and sometimes it does not - sometimes it works standing still and sometimes it does not - I can hear the clutch engage when I click the air on.
When the air starts to cut out I hear the fan (under the dash) slowly wind down - barely turning - If I start the truck with the air "off" and then turn the air "on" it will sometimes work - other times I'll just be sittin there and the air will blast on because I had left it on full the last time it worked!
So you have a problem with the blower motor, not the temperature of the air coming out?
Are you having trouble with the blower motor slowing down and not blowing as hard, or does the air always come out at full speed?
Does this only happen after you have been driving for a while with the a/c on, or is it something that can happen when you just get in the truck and take off?
Does the amount of condensation dripping under the truck with the a/c on seem normal, or does it seem like maybe not as much as in the past?
I have to wonder if you have a failing blower motor. If it was slowing down after the a/c had been running for a while I would suspect that the evaporator was freezing up due to condensation that couldn't get out quick enough, but this shouldn't be an issue if it happens shortly after starting.
What I would do is use a headlamp or fog lamp bulb to load test right at the blower motor connector when the motor has slowed down. I would unplug the motor and use the bulb and a couple jumper wires inline plugged in just like the motor would be. With the blower motor on if the bulb lights just as bright as if connected to the battery but the motor is running at a slow speed, then you can condemn the blower motor.
so basically if there is power to the motor but its not running its the blower motor gone bad - what if the bulb doesn't lite up?
How do I access the blower motor?
The blower motor is mounted to the bottom of the heater box on the passenger side, easy to see.
If the bulb doesn't light up at all then the motor wouldn't run. If the bulb lights up dim then it's getting a poor ground through the resistor and control head, or a poor power feed from the ignition switch and blower motor relay. You would need to determine if it's a power or ground problem using your load test light and then check into the appropriate side of the system from there.
Ok, I'll start there - so you dont think it has anything to do with the vacuum system?
do you have any idea how much a blower motor will run?
If it was a problem with vacuum it would change where the air comes out when you accelerate and decelerate - it would switch to defrost on it's own when you accelerate and then back to the position you picked when you let up on the throttle. If you had no vacuum at all then it would just default to defrost all the time.
The blower motor will start at about $45 aftermarket, a bit more expensive at your dealer.
You're welcome, and good luck!