1999 Isuzu Trooper Radiator not taking coolant back from re

I had my radiator changed approx. 2 years ago. Since then the coolant will go into the reservoir and not return to the radiator. I have had it checked several times, had hoses replaced, pressure tested, etc. The only problem found are some pin holes in the top of the radiator. I have a vented cap now, so I can pull coolant out of the reservoir and put it back in the radiator. The coolant level in the radiator drops about 2 inches in a 100 miles. Any advice?