Timing

1965 Plymouth Barracuda 273 Formula S HI-PO
New Pertronix D144600
New Edelbrock 1405 600cfm with spacer
New Blue Streak 8mm plug wires
Cleaned and regaped plugs
Engine was rebuilt before I bought it, ported and polised, new oe cam and not sure what else.

My question is the timing jumps 10 degrees at idle ( 700 rpm)
I've sprayed carb cleaner around the base of everything and no signs of vacuum leak.
I've adjusted the idle mixture screws as best I can.
Timing is set at 10 degrees BTC with vacuum disconnected.

Any idea why I can't get the timing steady?

Tried to give as much info as I could.

Thanks Mike