My 1996 Volvo 850 ac is acting funny; I had it recharged and it now cool on and off. if it sits overnight it will blow cold until the next time I shut off the engine, after restarting it will only blow hot air.
I noticed the low pressure line and the cylinderical tank in front of the compressor are freezing up on the outside when the ac is working.

what is the problem with this ac, please help!

Sounds like the exactly same situation I was having when I was trying to fix my A/C. ALthough since then (prior to a complete fix) the compressor clutch or climate control box has died so I have no A/C at all now.

Best I could tell is that the A/C was simply blowing too cold. The place that got it working again, was amazed how cold it was, and they told me it was blowing at 32-33 degrees, and it seemed to be freexing up by the way it was acting.

Only difference in how mine worked and how you describe your symptoms, is that mine would quit blowing cold after about 30-40 mins of constant use. Although it would work fine after sitting over night, it would also work fine if I turned the car off for about 4 hours then tried again.

As for what was causing it, I have yet to figure it out. Possibly a clog somehere in the system causing it get too cold. I dont recall what part is what as A/C systems are not my thing, but I have been told to check the part under the passenger side dash (evaporator I think), and the one in the front engine compartment below the radiator on the passenger side (cannister of some type that can only be replaced not fixed, as I have been told). Neither item is cheap.

It is possible that your evaporator is clogged, causing the block in the looping system. When the looping gets clogged, the cooled gas cant get circulated, (and in the expected way how it is supposed ot work) "WARMED", before it goes back into the unit and gets cooled again.

Is there a way to test and see if the evaporator is clogged or I should buy a new one?

Thanks, I will have it checked by a volvo mech.