RPM- Iddle issues

[shermro99]

1970 C10 p/u 350 5.7L with a performance cam, edelbrock 600cfm carb, Timing set at 12 BTDC.

My issues is this:
In park idle is runs about 900 rpms. I let it warm up to optimal operating temp and once I shift to reverse the rpms dramatically drop to 300 or so for a bit before it comes up to 500. Once I get going it runs well. BUT when I come to a stop light the RPMs drop to about 300 and the truck sounds like it wants to stall before it kicks itself back up to 500 rpms.

I have checked timing, vacuum leaks, accelerator pump, jets, springs, spark plugs/wires and so on.

I am at a loss as to what I need to do next. PLEASE someone help.