The short answer is no, it's not common for lines to be clogged on these systems.
Let's try to figure this out, I'm sure someone will let me know if I make a wrong deduction.
IF there is sufficient refrigerant (Note IF in capitals):
The compressor turns on when there is pressure (>25 psia IIRC) and off when the pressure falls below this. As you deduced, blockage could be a problem. The most likely point (IF you have sufficient refrigerant) is the orifice. This is a inline "valve" inserted in the line between the condensor and the evaporator on the passenger side, it has a nut on either side. The orifice is surrounded by a screen that can get clogged if there is a failure in another part of the system. I understand there is also a screen on the input to the evaporator, but the orifice will clog first. Moisture in system can cause ice to form at the orifice causing intermittent problems. Look at or feel the orifice if it's hot on one side and frost (or condensate on the other, it's blocked. - This can also be an indication of low refrigerant charge (remember IF there is sufficient refrigerant).
Look elsewhere on the system, presence of frost or condensate indicates a high pressure to low pressure transition (partial blockage).
Another problem could be compressor going bad - bringing up to enough pressure to turn on compressor but not enough to cool.
Before tearing things apart, read stickies above, buy/borrow gauges (Harbor Freight - 35USD) and temp gauge (5USD Autozone), read pressures and vent and ambient temp, and post here.
Ask questions, everyone's friendly.
