Troubleshooting help please

Sure sounds like the volume control. You mentioned it does not happen with your other preamp. Do you have a second system you could put the questionable preamp in. That would tell you for sure if it happens there too.
Have you tried maybe spraying some tuner cleaner in the volume control?

The gain in your preamp could be too high. I have had this happen a couple of times with different preamps. You may be able to get it adjusted, which is what I did.

Good luck.

If the gain is too high, I assume that preamp could then work fine with another amp and that it is simply a pre-amp & amp mis-match issue...is that correct. I will spray contact cleaner just in case.

Thanks

If the gain of your pre-amp is too high you would not have very much use of the VC. Assuming a normal amp input sensitivity, you would probably be restricted to volume control use below 9 o'clock where some VCs may have balance issues and, if stepped, the incremental increases between steps is too high.

For example some pre's have 3db (or more) steps at the bottom of the scale, but when you get up to say 10 to 2 o'clock (the former being probably normal for average systems and the latter for really inefficent systems) the gradient of steps may be reduced to as little as .5bd and gives you a lot of flexibility for fine tuning.

This is sort of generic information, but I hope it helps a bit.

Why would this be a problem? I would not consider that to be something that required repair. I would say it is a result of of the gain structure of the preamp. Don't worry about it.