Weird that you can't get a stable engine run between full advance/retard. There "shouldn't" be a problem with cam timing unless it's sitting in a barn for a reason (like someone else couldn't set the cam timing correctly and gave up) If the belt is out a tooth it will cause chaos. And now for something really annoying - distributor out of sync (test this if you have confirmed that everything 'appears' to be set up correctly - timing belt and distributor install)

When you say you can't get a stable idle are you #1 setting the distributor and full advancing and it shows a sign of life then #2 bumping the distributor a tooth forward and fully retarding the timing and getting a similar result? There is a weird phenomenon where for some reason the distributor is out half a tooth. I have run into this myself and it resulted in lots of colorful language, high blood pressure and a few hours wasted trying to figure out what I had done wrong even though I know Mitsubishi engines and did it text book style. I solved it by swapping the firing order of the leads from #1 and #4, then #3 and #2. Set the distributor alignment mark where the retaining nut is located - and it fired and ran first shot. Don't have a clue why this would happen but it had me stumped until I tested my theory out.