Dore,
Don't worry,with proper chelation plan you'll get below 1000 again,and yes if you can't take the high dose continuously,taking it twice or thrice a week should be helpful.
Its a bit tricky that why your hb is unstable although you are going for tx every two weeks and getting the same amount of blood,do you check the dates on which the blood is collected from donors,the fresher the blood is the more it stays in the body,have you ever tried increasing you tx intervals and the amount of blood,like you can try getting 2 bags every 4 or 3 weeks,it would be less visits to the hospital and may be your hb will remain stable ? but its my opinion only,you have to calculate yourself whats best for you.
Good luck,
Zaini.