Some questions on late game play

I've been struggling with late game play recently. I usually play a very tight early game which often leaves me with around 675-625 by the time BB is 100. Once here I feel that I either play too tight, and end up having to go all in when I have 315 chips and pay a BB of 150, or I play too loose, earning some chips from pushing but then busting out because I got called by a better hand. The following are some situations I often run into. What would you do and why? Thanks!

1) There are 6 ppl left. You are BB with 650 chips before paying BB of 100. You get dealt Q6o and it is folded to SB who has 1250 chips before paying SB. SB raises to 300. What do you do? What if you had K9o?

2) Same situation as above, except you are now SB and the 1250 stack is BB. You have Q9o what do you do?

3) You have 615 chips and in the SB. BB is 100. There are 5 people left. You have AJo. CO raises all-in.

Thanks for you comments.