Help! A/F Lean!!

[LevinGT]

Okay i have this problem; when engine is cold mixture is rich, which makes sense, while driving A/F goes up and down. Still makes sense.
But then, as engine warms up, after say 20 min. at WOT A/F goes lean @ 3000 rpm till redline?
But only while driving, then as you stop and idle still lean, after 1 or 2 min idle A/F goes stoich. When revving A/F goes from rich to stoich, and everything seems normal then.
It's just a simple A/F meter, so I don't know how lean it's getting.
Any ideas?