First Post-Restoration Glitch

[compucat]

My recently restored Zenith roundie has developed its first problem. The vertical is acting up. The vertical is non-linear, won't stop rolling and briefly collapses to a line for a fraction of a second. Everything else seems to be alright with the set. I replaced the vertical output tube during the restoration and the set has been working perfectly up until now.

Having restored many antique radios, I was anticipating some kind of near term failure as is common when a forty plus year old piece of electronics is back in regular service. The vertical circuit does not look too complicated. One thing comes to mind, however, there is a black beauty cap rated at 0.015 uf, 1000V that I did not change. I had no replacement for it and since the set worked fine, I left it in. It is located in the feedback loop from the vertical output tube to the oscillator. The way the set is acting makes me very suspicious of this cap. This set does not seem to use hardly any of these caps, I'm surprised there are any at all. Is this a likely culprit? Any advice from experienced roundie restorers would be appreciated.

Up until now, this set has been absolutely wonderful.