AC line obstruction common?

[paramax55]

It appears that either I have an obstruction in an AC line, or I have a sticking expansion valve. (15 psi on the low side, 300 psi on the high side, frozen condensor). The AC has been working fine for the 5 years I've owned the car and I've never opened the system to let in any contaminants. So, if it's junk in the lines, the compressor is the only place I can think of for it to come from.

I know I need to flush it out and probably change out the valve, but I've never actually had an obstruction - even with complete compressor failures. Are E53 systems that fragile, or have I finally just gotten (un)lucky after 40 years and countless cars?