AC problems

[emarker]

I have a 2010 2500 and I discovered that the evaporator was leaking so I pulled the dash and installed a new one. When I recharged the system the low pressure side will pull down to around 20 with the compressor running and the high will jump up to 450 then the compressor will turn off and the low will go up to about 50 or 60 and the high drop to about 300 then the compressor kicks on and it starts all over. I've gone through 2 new expansion valves and blown down the lines with nitrogen before vacuuming to check for blockage and can't find anything. Any ideas would be appreciated.