Alright so here's my problem. Rebuilt engine with 90 miles. Drives great. Starts great. After about 10 minutes of driving when I push in the clutch and brake at the same time idle will drop to about 400 and pulse. If I am rolling and I push in clutch idle sits at about 1000, if I park it and stick it in neutral, it idles at about 1000. Only bad sometimes when rolling slow and clutch in and braking. Please help! Tried adjusting idle screw didn't help. Can't find any vacuum leaks. Gears grind sometimes.

 

Also seems to run super rich, pops tons on deacceleration.