Randy,
I had the same issue with some 14 year old System Three epoxy. I made test batches from every combination of old and new I could think of. All mixes set up, but cure times became unpridictable. Some mixes did not set up very hard. And the older mixes usually were discolored. I concluded that old with old and new with new was the safest bet. Also, if the old epoxy has experienced crystalization (even if thawed) it will contaminate the new, and cause it to cystalize easier.
Jerry