Nakamichi ZX-7 Cassette Deck - - - Dubbing Problem

So I just had the pleasure (hopefully) of purchasing a fairly high-end system from a well respected bandleader/composer 2 weeks ago and have been busy in my limited free time testing out all the equipment. Unfortunately, I have come up with a number of issues through the testing process. This question relates to a Nakamichi zx-7 cassette deck.

The play function of the deck works perfectly, however, dubbing is a bit of another story. When dubbing vinyl yesterday, I came across a problem with left channel being a bit quieter upon playback than the right channel. The levels are the same when listening through the “source”, however, when playing back the recorded tape, the left channel was noticeable quieter than the right by about 10 decibels. I tweeked the “sensitivity levels”, turning up the left side and down the right, but this time when I played it back, the left channel was completely dead, the right was fine. I then recorded again and didn’t move the setting at all and upon playback, the left channel had come back but was still a bit quieter than the right.

It seems to be a bit of an intermittent issue, wondering if anyone has any ideas on what might be causing this problem.