different retention times on repeat runs of the same sample

[officed144]

I am an newbe at GC so please bear with me.
Here at work we have an older GC with a packed column and a FID detector that I am trying to use to quantify some additives and contaminates in our mill rolling oil. The peaks I am having trouble with are from an additive - lauric acid. I see the peak fine, but the retention time isn't always the same. For the duplicates of the sample I was running today, the peaks were sometimes 10 sec apart which is playing havoc with my peak analysis efforts.

I inject the sample manually, but I know I have less than 1 second variation in my injection technique - certainly not 10 seconds.

My heating profile lets it set for 5 minutes then it ramps up evenly to max temp over the next 25 minutes. Is this something I should change? Or is there somewhere else I should be looking to solve this problem?

Could some of you more experienced folks give me some ideas on how to reduce the variation in retention time?

Thanks,
Ron

[aldehyde]

what solvent is it in, injection volume, and flow rate? what are some of the other analytes in the mix? are there any other retention time problems (ie, are the other peaks eluting consistently?)