I don't know what could be causing the problems in all honesty. When I first got into water cooling I wa running a maze1 3/8 barbs, big momma radiator with 3/8 barbs, and an ehiem 1046(80gph). I was running a duron at 1.1gig at 1.85 volts. My temps never went over 35c.

My guess:
a combination of: poor lapping, a jammed/dirty rad, and a weak pump.

Your pump (Eheim 1046) probably had a better head than that Rio 180. Your block is definitely properly lapped, and you had a new rad (right?).

The ViaAqua 1300 doesn't have much of any fittings at all - just some crappy smooth plastic openings, and you're probably right that those are 3/4", I've never measured them. But my ViaAqua has 1/2" brass fittings on it, and it was shipped to me that way when I bought it from CPU-Watercool (who's www-page seems to no-longer work). Judging from the looks of the pump, those fittings screwed right into the plastic openings with little or no modifications to the pump (they probably tapped the holes, there are no rough edges from cut plastic though - and then screwed them in with a bit of epoxy on the threads)

My Via Aqua came with 3/4" barbs installed in 1/2" NPT openings. NPT thread sizes are for the inner diameter - if you measure the threads, they will be closer to 3/4".

My 1300 came with barbs, and all kinds of adapters in the box (all of which may be useful for aquarium use, but useless for WC). I substituted the 3/4" barbs with 5/8" OD, 1/2" NPT barbs with some thread sealant - works great.