does the machine crackle with its own recordings?
do its recordings play ok on other hifi machines?

I suppose from the fact that its gone away the problem may have been an
oxide deposit somewhere around the heads, which whilst not enough to cause
full snowy pictures etc, is still sufficient to reduce the head output so
that the hifi track drops out. With use the deposit has 'worn' itself away.
if this is the case then your machine should be ok from now on, (except if
you have an iffy tape dropping oxide of course)

If the machine was new theres always the possibility that its 'bedding in',
particularly the felt brake/back tension pads.

If it still has problems playing its own recordings then as mentioned before
the 'switching point' may be mis adjusted. This is the playback setting for
switching between the 2 heads spinning round, and is (on modern machines) an
electronic setting stored in memory. Sometimes you can see the effect of
this at the bottom of a picture when you see the last line bobbing about-
when the timing cuts off a little too early.
This setting is adjusted with a special engineers remote available from JVC.
You can't adjust this yourself, you also need an oscilloscope to measure the
results.
If your machine needs this setting tweaking a JVC centre should be able to
sort it pretty quick.
Of course if the machine is new and set wrong, you should be asking
questions!!!

I'm in the UK and its interesting to hear from others that you've had
problems with Hifi /tracking over in the USA, we get very few Hifi or
switching point faults for that matter.

Where are your machines made- Europes are made in W Germany. Perhaps its a
site related problem?

Andrew