Compressor Lockup

[adrenochrome]

When my dads 2000 Windstar's a/c stopped working, I figured it all leaked out and just needed some stop-leak and freon. But of course not, it just happend to be leaking right out the front seal of the compressor. So I ordered a brand new compressor, accumulator, and orfice tube. Compressor came dry so I added the correct amount and type of oil, installed it, vacumed the system down with no leaks then added freon... Done. It worked great for about 8 months... lol... Then I get a call saying the compressor locked up and it literally stalls the engine when he activates it... Im bummed because there was only a 3 month warranty for the cheap one I bought which was a Visteon if im not mistaken... So im kinda back to square A again if not worse. It still has all the freon in it and it does stall the engine when you switch the a/c on... Im figuring that either the stop-leak that has been added (I believe many times before I did) caused it or there was contamination in the system because I refused to flush it... He did say a few months later it wasnt as cold as it used to be so he should've just brought it to me instead of adding another can of freon. I read later that on Windstars you have to replace the condensor because thier design is nearly impossible to flush thoroughly. Should I just replace the compressor this time and hope that the last compressor was just faulty and what are the main causes of compressor lockup?