After 20 min. it starts to sputter and pop and starts missing
When it's cold it runs fantastic
Does this mean I am about to drop a valve seat or is dropping a valve seat a gradual thing? Is it a done deal once it happens or does it take time?

lets not jump to conclusions. If it runs great when cold and crappy when hot, you could have a vacuum leak, ignition problem, FI problem, etc.

Start with the less complicated (and less serious) scenarios first. You will need to become familar with the archives here on the site. Search and learn.....pelican tech articles too.

When a valve seat drops it generally happens fast. There is a lot of thing that can cause those symtoms, but quite often it is as simple as a tight valve. Re adjust
them all to make sure, before spending money.

Check for vacuum leaks, check yor coil too

What kind of coil do you run? I had a Crane Fireball once. The car started to miss and by the time I got home it had gotten very soft and was melting. It was missing more and more but I made it home. But when it was cold, it ran better but when it go hot again........it melted more.
Check valves and vacuum leaks too.

I agree with the valve adjust, probably geting too tight when hot and they do not close fully.

Are you running FI? There is a temp sensor in the FI system that could be screwed up. I would do a complete tune on the engine. Once a valve seat is dropped I would think it would run bad all the time and wouldn't be temp dependent.

bosch coil, petronix on 914, looked to have original coil on it. Putting new Bosch blue coil, starter and compufire on Mom's Super Beetle tomorrow, thought about it and figure it has the original ignition coil, 28 years young, starter is about 15 years old

Still have Injection? Is your exhaust tip real sooty and black? Could be the cold start part of the FI is still running it it rich.

Something else to consider if you are running carbs is the idle jets could be clogged. If you are running carbs take the cover(s) off and hold the choke almost but not completely closed when it starts running rough and see if it clears up. If so dissasemble and clean. Just a suggestion from personal experience

Mine did this once. The CHT sensor was loose. Tightened it back up, ran great.

Had a car do that once....

Ran great when cold, 5 minutes later ran rough with black smoke....

Every day on the way into work and at the same spot...

Finally traced it to a spark plug wire....

Darned wire measured good when cold, and became a non-conductor when warm...

Hope this helps...

Ken

Sounds like it is too lean when warm. As it heats up it gets leaner. Or something electrical is getting warm and not working correctly. Once had a ignition condensor that failed when it got warm.
You should tell us what set up you're running and if it's been correctly timed. Did you just get this car and it's doing this or was it running fine for years and now it's fuchen up on you?????

How are you doing with this? Any progress?

My experience with this problem is that 99.9% of the time it is caused by valves that are adjusted too tight. The first time I saw this I was amazed at the transformation that a simple valve adjustment makes. There are so many other things that are affected by improper valve adjustment that it is certainly the place to start before you try other things,especially before you spend any money! If you do it yourself a valve adjustment is free!