What are your temp program parameters?
It almost looks to me like you have something sticking around on the column that's coming off at higher temperatures.
I know you said that you tried baking, but my inclination would be to do it again at a temperature 10-25º higher than the highest temperature of your run(assuming your column will allow that) and leaving it there at a high flow rate for a couple of hours. I'd also try solvent flushing the column, but I've also been looking for excuses to play with my apparatus for doing that
Have you injected any crude samples into it? I'd have my concerns over the tars and other very heavy components sticking around. They SHOULD in theory stay on the liner(although it's also worth checking/changing the inlet seal if your GC has one and it hasn't been changed) but could also migrate into the column. If that's happening repeatedly, it might be worth dropping your inlet temperature a bit(if possible) to keep the heavy stuff from migrating to the column.
Also, you don't say what solvent you're using. When I've done SIMDIS in the past(on biodiesel in my case) we prepped everything in CS2. That can be a bit of a cantankerous solvent. As part of troubleshooting, I'd probably try injecting a different solvent with as close of a BP as realistically possible(maybe pentane, or possibly DCM) and seeing if it gives the same result. That can rule out possible solvent contamination.