You didn't say you readjusted the timing after replacing the chain. That chain also drives the distributor. If it had 6° of stretch on the left (driver's) cam, then it's a fair bet the distributor was set at least 3° more advanced to compensate. After putting the new chain in, your timing is probably late (later timing -> lower idle). Check that first, it may be the easiest (but not most obvious) answer.