Session randomly disappear

I learnt that upgrading to 3.x will fix the problem but that will be a huge project to us and given the framework has no other issues other than this we would like to avoid upgrading it as much as possible. 

It has a number of security vulnerabilities.
The 3.0.0 changelog contains hundreds of bugfixes against 2.x.
If it didn't have issues, new versions wouldn't be released.

Hi Narf, i agree with you on "If it didn't have issues, new versions wouldn't be released.". But the fact that we are still using this version despite this version which was at its end of life 1.5 years ago means this is a rather stable version and we are happy with it. Other problems this framework has either doesn't affect us or concern us. 

As a programer I understand it is a good practice to keep the framework up to date and im all up for it if i got the time and resources; but at the same time i have to consider the cost of my decisions and all the alternatives.

You've cherry-picked the most banal part of my comment to argue against, and that is simply because you're looking for a reason to avoid upgrading.

There's a 99% chance that at least one of the security flaws fixed in more recent versions affect your application(s), so I wouldn't wave-off that part as easy as you're doing.
And if CI_Session is your immediate issue ... The 2.x version is fundamentally broken, and rewriting it was a major part of the work towards 3.0. Knowing all the issues it had, I'm almost certain that you don't have a developer capable of "fixing" it; and if you did - it would take them much more effort than it would to simply upgrade.

Depending on your application size and how skilled your developers are, it may take up to 8 hours to upgrade, which you've already spent waiting for non-existent solutions here. Some people on this forum have done it in less than an hour.

Plus, don't underestimate the political/social effects of your decision - no developer wants to work on an outdated project, let alone one that's been deliberately kept that way. The last time I left a job, it was exactly because of this.

Hi Narf, Im not trying to start an argument here. I dont mind waiting as we got quite a lot of works here to do while we wait. I just thought it would be nice if i can avoid upgrading thats all im saying. 
I understand that 3.x is better than 2.x but as far as the scope of our project goes, we dont want to spend time on the things that we dont need just to follow good coding practice. That is how business runs, not based on good coding practice but on needs. I do battle for more resources whenever i see a chance so we can finish projects properly but that kind of luxury can not be expected all the time.