I have an old General Controls millivolt gas valve:
Https://picasaweb.google.com/112652995081934659158/GardenHeating#5515080849589233618

The valve is starting to operate erratically (sometimes does not come on until I tap it).
Can someone recommend a replacement? Thanks.

I would start by replacing the thermopile. When they get weak, it will cause problems with gas valve operation.

I agree with mygirlsdad77. A weak thermopile or a dirty pilot burner will give you a erratic gas valve operation. Its hard to say with out testing the millivolts. Yes millivolt gas valves and thermopiles are still available.

I guess that is worth a try; but the current thermopile is at most 3 years old (possibly less).
The valve is probably 50 years old. I would like to have a spare valve available in any event.