If they get better when setting the FSX internal limit to a fixed value of e. g. 25, the "unlimited" setting is the culprit there.
You could only help the PC with defragmenting the disc and try some lower settings, so his workload includes more of the texture processing when set to "unlimited". With this setting you actually tell him to drop any optics for the winning of fps, so he just does like he should, in a way.

Now the trick is to find an external framelimit which still allows him to take enough time on textures while staying on "unlimited" on the internal limiter of FSX.
Sounds weird, but he actually is faster when his internal limit is set to unlimited, so you want to avoid any other setting there and go with an external limiter then.