My 2001 Thinline is suddenly starting to go sharp on the bass strings when I play it. The treble strings tend to go flat a bit under heavy bending - that's normal. But it's having trouble holding tune when I play it for even 10 minutes. Checked the bridge intonation and it's dead on (Wilkenson compensated saddles). Could it be a neck stability issue? If so, how do I fix it?