i can change voltage in bios bot only between 1.6v and 1.95v................. no other options and my ram needs 1.5
Your RAM will be fine running at 1.6 volts.
As far as instability is concerned, about all you can do is test by substitution until you find the problem. A PC similar to your will be invaluable.
Start from the core system - PSU, motherboard, CPU.
Don't assume anything.