Robert,

There are a few basic reasons that the signal can cut out.  Here are some (not in any order)...

1)  It's THAT weak.

2)  The squelch is too restricted.

3)  If it uses CTCSS and it's weak, the receiver may not capture enough audio to allow an open squelch (try disabling CTCSS).

4)  A local transmitter on/near the frequency wanted is blocking what you want to hear.  This may be simply near the frequency of interest but strong enough that the AGC (part of the receiver chain) dials down the sensitivity (it can't manage signals too loud otherwise).

5)  The bandwidth of the signal may exceed the setting of the receiver, i.e. you're listening in NFM but the signal is WFM and it's going out of the bandpass of the receiver.

6)  Local noise sources (LED lights, neon lights, wall wart power cubes, many electronics) may cause interference.

If there is a pattern to the dropping out, that may assist in knowing the reason.

One quick test, try another receiver and see if it continues to drop out.

A pre-amplifier at the antenna may boost the signals (and noise sources) well enough to help, BUT if you're also hearing very strong signals, it can be overdriven too (swamped is the usual term).  It can also be useful to overcome line loss in the antenna feed (which is why you put a receiver pre-amp AT the antenna).

If it's something you need to hear more consistently, a directional antenna may help (at the cost of less hearing in other directions).  Using a directional antenna, one 'listens' in that direction and they tend to null out noise sources in the other directions.

Using a higher gain antenna for that band may also be useful.

Good luck,
Rick NK7I