1.8 L not running well Help, bucknig after a 10 min drive or warmup Options

[Heeltoe914]

I have a 75 1.8L it runs good for the first 10 min lots of pull and power. after about 10min it starts to buck on off rpms dont change, But it hits a big dead spot. and if you keep going it just gets worst. If you puch the motor over 3500 rpms it gets a little more normal.
Now get this let it sit for a cooldown lets say 30min, you start it up and go for about 10-15min, and than its back, Bucking very consistent. It feels like its running out of gas but its not, and this is the same problem 5 times in a roll now.
I did a vavle adj, checked points and wires and plugs. but still no differance. changed out coil. checked with carb cleaner for vac lecks. and checeked vac at vacum advance on both sides of Dist, Now I am a little lost, I now 1.7 and 2.0 but this is my first time owning two 1.8L.
Will take any tips that come my way. Thanks