66 zenith high voltage dies after 30 minutes

[ctc17]

I figured id post this looking for ideas before I get any deeper into it. Its a combo and these thing are not easy to work on.

25mc36 chassis 1966 color.

After 30 minutes the picture gets yellowish and in seconds fades away. The high voltage drops to nothing, then slowly comes back up but only to about 15kv. Once the set completely cools off it will work great for another 30 minutes.

The HO tube drive remains the same as does the cathode current. I removed the HV regulator tube, no change.

I have subbed the HO and most of the others in the horizontal HV circuit with no change.

Any thoughts or experience would be appreciated.