Air Conditioning - a/c freezing up w/fan in auto position

i've had a tech out here twice, first time he put about a pound of freon in.
it worked for about a week and then froze up again.
then it started freezing up about twice a day.
when it happens i turn the fan to ON and thaw it out and set it back to auto and it's good for about another 12 hours.
my air filter is clean, my evaporator coil is clean in and the condensor coil is clean outside.
all my vents in the house are open.

i have the tech come back out again. he can't find anything wrong. he checked to make sure the a frame coil was clean and looked down in my ductwork to make sure no water had accumulated that might block air flow, he also said i could have a bad contactor on the outside unit and checked that as well (all looked good) he says my pressure was good as well, it was 75 and he said for the temp in the house maybe it could have been 76-78 so he adds maybe a half a pound of freon just to top it off he says and gets the pressure up to 78.
i ask him if i could have a freon leak and he says most likely no because the pressure is good.

so anyway, he leaves and about an hour later it freezes up !!
now, i notice that everytime it freezes up when i go in the garage my evaporator fan is NOT running, but the outside condensor fan IS running.
the tech said that if the outside fan runs and the indoor fan does not run then my unit will freeze.

so i decide to do a test and run the a/c with the fan ON for a couple days and notice that the unit never freezes. i switch it back to AUTO and it freezes again.

so wtf is causing this?

either when the a/c is in auto the evaporator fan just decides to stop running and that causes it to freeze.
or the unit freezes up in auto and then the evaporator fan has a sensor on it that shuts it off when the unit freezes.
or, i have a freon leak or some other problem, but whatever it is the evaporator fan is always off when it is frozen

any advice, this is perplexing...

Is the outdoor unit shutting off when you put your stat in the off position?

yes it is.
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From afar, it sounds like a bad fan relay in the furnace/air handler. If it will run in continuous, that says the motor isn't dropping out due to failure. Depending on the age of the furnace, could mean a new circuit board or if older and doesn't have a board, just a change of the relay. I'd call the company that did the work back and let them know your findings.

tinmantu, could the t-stat possibly cause this? the tech recommended if it froze up again to change the t-stat. i thought maybe it controlled the evaporator fan and was telling it to cut off at the wrong time maybe? what does a fan relay do?
thx.

you could eliminate the stat by jumpering r and y and see if it drops out after the usual time....the purpose of the relay is to determine whether the system is in heat or cool mode...older furnaces, relay was set for heating mode first, when energized the relay switches to cool mode and a different blower speed. If you start messing with wiring, make special note of where everything was before and know if you touch the wrong wires, you could create more problems.

I might want to get the freon re-checked, maybe by someone else preferable using superheat/subcooling. The only way you could have lost freon is with a leak, so you don't want to be overcharged if there really wasn't one.