I twisted my right knee some years back and has given me problems every since! it sometimes buckels without warning and it pops, and can't straighten?

You most likely have: A meniscal tear and all your symptoms, that you describe, are typical for such a problem. The knee buckling under you is a dangerous situation, as it makes you fall and can result in broken bones or other injuries. You need to see an orthopod to get evaluated/investigated and most likely will require a scope surgery to be treated.