Three fouled plugs three fouled plugs see how they don't run!

[92Rustang]

I'm driving home last night about 65 had the radio up. The car starts missing BAD. I stop and see what I can see. Everything looks fine, but the A/F ratio on the driver’s side is not reading until I gas it (little tap). Then it spikes to rich and down to nothing. The pass side looked a little rich for driving.

I just put this gauge in and have about 40 miles on it.

I pulled the spark plugs and three were fouled (4,7,8). I pulled the upper intake to check the rockers and the intake had oil around the ports for these cylinders.

I think this has to be the problem, but how do I stop it? The intake was tight and the gasket newer. I have removed the intake a couple of times to adjust the RR. Can I use a little gasket sealer on this or do I have to buy a new gasket every time I remove the upper intake? And this did not happen over a couple of days it was like a switch boom missing. Thanks for the help again