Reset Timing?

hey whats up once again lol, ok well on my toyota supra turbo 87 M/T i gots me a small idle problem i think its the timing. well it likes to idle at 650 rpms for awhile before it drops to 500 like every two seconds before it drops way to low and dies, well my timing is set at 12 degrees advanced thinking that would be best since i use a 92 oct gas all the time but would it be better just to set it at 10 degrees advanced instead? although i did just replace the head gasket and pretty mch every last gasket and oil seal on the carit shows no signs of a vacuum leak anywhere and i can drive her around but it dies if it goes to an idle after that also i have no power loss when i drive around it runs like it did before just alil better besides the stalls heh..

any thoughts please let me know