Help with fouling plug

This is on my 97, 350 vortec.
I while back installed new AC platinum plugs and found #5 was encrusted, with hard deposits, could barely see the electrode. With the new plugs it ran great for a while and then started missing again under load. Checked the plugs and #5 was starting to build up deposits again. Changed that plug and again ran fine.
Started acting up again, and sure enough #5 is not working properly. This time it had some black on it.
The only thing I have done is swap igniton wires, which hasn't helped.
Pulled the spring off #5 and the seal and guide look fine.
Compression is 175 on all cylinders.
One other strange thing is that the plugs on the even side look like brand new. The odd numbers are ok but not as clean, maybe a tiny tiny bit of deposit.
So what the heck am I looking for??? I just don't know what to fix. Any ideas are appreciated.

My lower intake manifold gaskets blew, they got sucked in on #3 and #5 ports causing the plugs to foul out with oil. I was replacing #3 and #5 plugs every oil change to keep the truck running good. I'd be willing to bet if the LIM gaskets are original, that they are your problem. How many miles you have?