Hi Paul,
I went through the same thing - I tried standard Fel Pro, Cometic and a few others and everything eventually shifted and leaked. In my opinion, the only good intake manifold gasket for a street FE is the Felpro 1247S3. They have a steel insert and I haven't lost a drop of
oil or coolant since installing them. I also used the Motrocraft TA-31 RTV instead of the front and rear cork gaskets. This is what Survival Motorsports uses and I had great luck with both of these products. You can do a search on the Fel Pro P/N and find a few threads on this site.
Here is just one from the FE Forum:
332-428 Ford FE Engine Forum: Which gasket would you use? Felpro 1247-S3 or Mr Gasket 202A?
This is not an easy job so you don't want to do it too many times
Good Luck.
Kevin