Tube Issue Blue Flashing intermittent sound BFSR

[Rbuzz]

This problem developed during a gig in my friend's 66 Fender Super Rev.

There had been an issue with his rectifier tube. The amp would blow fuses and some arcing in the rectifier tube was seen by the keyboard player seated behind the amp.
We swapped his power tubes and the rectifier with two brand new TAD 6l6GCs and a yellow jacket 5AR4 substitute.

The amp was fine for several practices and 2/3rds of a gig then the amp started this behavior as seen in the youtube video I made (link below).

I found a cold solder joint and reflowed solder at that junction of the .1 uF cap and the 220k resister that divides the bias neg. voltage. That didn't fix the issue.

The engineers at Peavey in Meridian MS did some work on this amp a few years ago but my buddy wasn't sure what they performed. (grounded ac plug was added).

I know the 470ohm and 1.5 k resistors on the power tube socket nearest the PI were replaced as was the 470ohm resistor on the other power tube socket (the 1.5 k on the tube socket nearest the rect. tube looks original carbon comp).

I removed the ground switch and installed a bias probe socket in it's place.
Someone also replaced a 100k slope resistor and .1uF cap in the tone stack of the Vib. channel too (not sure if Peavey did this or what the reason was other than those parts I assume needed replacement - including that blue molded 'ajax' signal cap).

I haven't seen this behavior in tubes, so before I start poking around more I was hoping to hear from others who may have gone down this road before.

(chop-sticking around the tube sockets didn't reveal any intermittent issues but it did reveal the cold solder joint I mentioned earlier which has been fixed).

I pulled the tubes for the vib. circuit and PI with no change in behavior.
With PI installed but no valve for vib. circuit the LDR didn't flash but otherwise no difference. Also if a guitar is played into the amp the bursts of sound includes the guitar signal in millisecond bursts.

http://youtu.be/Gyj5kJrhQ3Y

[TUBEDUDE]

How does the power supply string look? I had a 50W Fender with similar symptoms. Turned out to be an intermittent open in the inductor. Have you monitored the voltages in the string? Do the caps under the cover look ok? If they are elderly, one of them might be committing suicide.

[Rbuzz]

The power caps are all original Mallorys so an intermittent open cap could be the issue since they have about 48 years of mileage. Would a continuity test on the north and south poles reveal this issue? How shall I proceed other than visual inspection under the cover?
Thank you for the response by the way. This amp keeps throwing me curve balls. Every issue I solve a (seemingly) new one pops up.

[Stevem]

All the electrolytic type caps in the amp need be sent packing at this point!
The caps in the output stage bias circuit need to be stepped up to 100 uf at 100 volts.
The screen resistors on the output tubes should be changed out for 3 to 5 watt wire wound ceramic types and take a really close look at the 1.5k resistors spanning each output tubes and many times they are not burned, but snapped in half to one degree or another.
I had a amp once where the output tube locating / centering pin had snapped off and get stuck in the socket and then the owner just grabbed another tube rammed it in and snapped the 1.5k resistor right in half and turned the already snapped off locating pin into complete rubble!!!!