I'd say it's probably the fan clutch. I understand what your mechanic said, but unless he (or she) put a rag in the fan or dismantled it, then it could very well be the issue. You HAVE to put an OEM clutch back on (Sachs) - do not use BAER or other Asian clones.

2cd guess is the radiator itself. They're a surprisingly easy replacement, and even the aluminum overseas units will give you a few good years if you get past 2-3 days.

3rd/4th guess - Unresolved airlock in the block, pump is really bad though shows no outward signs, thermostat is blocking everything, plastic bags from road blocking air entrance, condenser grill clogged, serp belt slippage, etc. And like Larry said, if it's not pressurized then the boiling point rises dramatically, thus water will boil and clog system at heat-points before it can get back to radiator. Caps are cheap

Good luck -