Just a update on this project.
My tuning issues returned after a while, seems like the saddle had a burr and the cheap plastic nut was worn through. ☹
I decided to take SingeMonkey's advice ( Thanks ? ) and ordered a aluminium Gotoh bridge and tailpiece set
(
http://www.stewmac.com/shop/Bridges,_tailpieces/Electric_guitar,_Tune-o-matic_bridges/Gotoh_510_Bridge_and_Tailpiece.html)
and Tusq Xl nut
http://www.stewmac.com/shop/Nuts,_saddles/String_nuts/TUSQ_XL_Nuts.html .
The bridge/tailpiece is a drop in replacement and no modification was needed. I ordered a slotted epiphone Tusq XL nut, it fitted the guitar perfectly.
Foster did the work on the guitar again, and I have to say, it was well worth the hassle. The guitar is a different beast now, more "airy" sound to it, tiny bit brighter and 100% rock solid tuning.
Only thing that is unchanged on the guitar is the pots and switches, don't know if I should mess with them, is it worth it?