timing stuck distributor in facing firewall pointed at number one?

hi, i just finished rebuilding the top end of my motor. Its bone stock 1996 Ford f150 5.0 302. i plugged the cylinder one with a cloth spun crank until compression stroke put balancer at 0 degrees stuck distributor in facing fire wall pointed at #1 did a 1/4 turn just to get it idling. its starting up and running problem I'm having i went to go time pointed light at balancer. I've never seen this before but it's bouncing from like 10 degrees to 5. I do not know what's going on, it's like bouncing left to right. Any help would be great, thanks.