In spring of 06' I had rockers installed so naturally got new valve covor gaskets. In the last week or so the rear valve covor just on the right side has started to leak pretty badly. The new gaskets were much befier than the OEMs. I looked for cracks in the valve covor but didn't see any and tightend the bolts (only 1 was loose). I am baffled as those gaskets looked like they were ment to last for decades not a lousy year. Am I missing something to check on? Could an underlying problem cause it to wear?

I'd pull the covers and clean the mating surfaces, inspect the gasket and if its ok then put them back on and follow the torque spec in tech info. AND then see if it still leaks.

The gaskets can and will over time flatten instead of staying puffy (above the level of the valve cover).

Check and replace as necessarly. Luck of the draw.