I ended up with a large radial crack on one of my exhaust valves once I disassembled my engine. The valve seat was also garbage IMHO. So I decided to take off both heads and have all the exhaust seats recut (they all looked like garbage) and replaced all the exhaust valves with manley inconel jobbies and replaced all springs (intake included) with manley springs/titanium retainers...as I'd assume the factory springs (soft!) wouldn't like the relatively heavy inconel exhaust valves when the OEM's are sodium filled lightweight stuff. Fortunately I do my own work and get parts cheap, otherwise I'd be rather upset, and I can easily see how this could be an expensive job to fix correctly.
I went with new OEM valve/cam/lash buckets instead of aftermarket shimmed buckets (supertech?); I only needed 6 or so new ones after recutting the seats...the lash on some of the intakes were out of spec too (even though they weren't recut).
Anyway...lame. Perhaps it's the awlful PCV system on these cars, sucking a ton of oil mist from the crankcase which eventually coats the exhaust seats which then doesn't allow the exhaust valve head cool enough between cycles. I dunno exactly. I have yet to add a catch can to my system.