Ughhh I need help. Tuning issues.

Ok guys I'm confused and need your assistance. I would like to say I'm pretty good at tuneing,but I'm a little stumped.
My 408 stroker with a QF 850 dp is giving me trouble.
So today I went to a buddies house and the car was running great. Outside temp was around 20 celcious. (68 for the American boys) and Still a little rich when I get on the throttle for the first little bit, but very happy and highway cruise was around 13.5. Afr. Idle was around 13.5-14.5. Anyways this is were it gets weird. Sun goes down and temp drops to 15 celcious. (59f) I flash the car up and idle is pretty lean. 15:5 16:5. Even when warmed up. So I drive away and the car runs as normal. Afr's are normal. I go to merge on to the highway and give quarter throttle and AFRs lean to 15:5 under load and the engine stumbles and misses. Let go of the gas and all is normal. Give really slight throttle and it's good. Give a little more throttle and rpm and leans out again and miss. .. I now come to a light and the car is still a little leaner at idle then it should be.... I don't get it. This entire time the fuel pressure is rock solid at 6.5.
I hope this makes sense.