[stevehertz]

Sounds to me like the value of the caps is too high. Generally it's not advisable to 'improve' on smoothing cap values by too much, especially in valve circuits which ok, is not the case here. But you're increasing the value five-fold? I'd stick with stock or maybe double at most. It may well be a 'quirk' of the circuitry that nine times out of ten the combination will work, but maybe it's a case of the exception proving the rule, as what you are experiencing?