One other issue could be scope parallax. Rest your gun on your shooting bags without touching it, then move your head around while looking through the scope - if the crosshairs move any on a target 100 yards away, then you have a parallax problem. Usually this just affects overall accuracy, but depending on how consistent you are sighting through the scope it could give you stringing. The parallax going out of whack on a scope has caused me considerable grief in the past before I determined what was causing me the problem.

Thanks to those who posted responses on what was causing the vertical stringing problem. I isolated each variable one by one and came to realize that the problem was the charge weight. I'm shooting 158 grain GDHP's over H110 with Winchester magnum primers. The manual that I used initially, showed a max charge of 14.5 grains of H110. Hodgdon's website showed a considerably higher charge - 16.7, if recall correctly. It also advised not to reduce the load by more than 3%, or inconsistent ignition would occur. With a charge of 14.5 grains, my rounds were 15% under max. I loaded a box of freshly trimmed brass with a charge of 16.7 grains of H110 with the same projectiles and primers and headed to the range this morning. With very cold hands, eyes tearing from the wind, and a lousy rest, I managed some three shot groups under two inches at 25 yards. With better conditions, I'm sure that I can improve upon that. Took some work, but I now have the kind of accuracy and power that I was looking for. Thanks again.