The electrolysis test doesn't really cost anything.
If it's a leaky head gasket or crack somewhere the excessive pressure would be stressing a solder joint. But I would think you see the radiator cap overflow spitting out a lot of coolant.
But I guess a slow leak could mess with the coolant pH. Seems very odd you'd have a slow leak/crack go on for what sounds like 6-7 years? I'd certainly look at the electrolysis first. Check it cold and again warm.
If it comes to it, you can do the block test yourself with this test kit:
http://www.superstreetonline.com/how-to/engine/impp-1108-blown-head-gasket-diagnostic/$25.97
http://www.summitracing.com/parts/oes-27...kyBMaAj418P8HAQNapa $42.99:
http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?R=BK_7001006_0361073538