Has gas pressure been setup on that valve? Is a safety dropping out coil voltage to the valve?
That’s a pretty aged furnace. Has the heat exchanger been checked out?

 

In the video, at the 3:25 mark, it looks like the gas just cuts right off, pilot to.

So I assume you have to keep relighting the pilot every time this happens?

Who installed the new gas valve you or a tech? When the gas valve was installed, was it adjusted properly with a manometer and combustion check done? Reason I ask is I know most home owners don't own a manometer and combustion analyzer with data print out, as they are expensive tools. So a lot of times the gas valve gets installed right out of the box with no adjustment.

 

Before I went into anything else I would replace the thermocouple.

 

Use a meter to see if 24v to gas valve is being dropped during continuous call for heat.

Either power to the valve is being dropped. or voltage from the thermocouple is dropping off, causing the main safety valve to close cutting gas to pilot and burners.

The pilot going out right when it cuts off - if it is the former, it may be related to draft through the boiler moving pilot flame away from thermocouple, dirty pilot orifice, bad thermocouple.

It's most likely not the thermocouple or it would happen when idling.

With a meter - can tell what's going on.
If the power is still being applied to valve when both pilot and main burners shut off, good idea to pull and clean the pilot assembly including the orifice. Can put a new thermocouple at same as as it's cheap and easier when assembly is already out.

If 24v is being dropped, messing with the pilot assembly won't fix it alone.

 

Time to get an adapter to test the thermocouple while it’s under a load to confirm if the the valve is bad or the thermocouple is bad when under a load…. Definitely not any secondary safety issue as they wouldn’t drop the pilot out…. My personal theory without being there would be either a weak thermocouple or the draft from the main burner is pulling pilot flame away from the thermocouple causing a drop in millivoltage from the thermocouple to the valve….

 

Definitely not any secondary safety issue as they wouldn’t drop the pilot out…. My personal theory without being there would be either a weak thermocouple or the draft from the main burner is pulling pilot flame away from the thermocouple causing a drop in millivoltage from the thermocouple to the valve….

technically possible but would have to be a coincidence with draft when it shuts off interfering with pilot - but very unlikely.
you're most likely right

 

Thanks to all that responded. Problem seems to be solved. After my post, I realized the thermocouple was just replaced along with the valve earlier this year so I wasnt suspecting it was the issue but.... I was also finding that getting the pilot to stay lit was getting harder and harder so I gave up last night and ordered a new thermocouple. New one arrived today and I tested both with a multimeter and sure ****, the one I installed in January was toast already. Will not purchase an ACE Thermocouple again. Installed an Eastman, $8 on Amazon.

Now my nest thermostat is 'offline' but thats another issue! TU!

 

Thanks to all that responded. Problem seems to be solved. After my post, I realized the thermocouple was just replaced along with the valve earlier this year so I wasnt suspecting it was the issue but.... I was also finding that getting the pilot to stay lit was getting harder and harder so I gave up last night and ordered a new thermocouple. New one arrived today and I tested both with a multimeter and sure ****, the one I installed in January was toast already. Will not purchase an ACE Thermocouple again. Installed an Eastman, $8 on Amazon.

Now my nest thermostat is 'offline' but thats another issue! TU!

Thank you for the update. Glad it was something simple. Nice job.

 

Glad you got it fixed.