Unsteady SPL Readings

If you are using a very cheap slm, (ie radio shack type), they are not designed to go that low in frequency response to get an accurate, consistent reading. It may help to get out of "a weighted" if you are in that mode as well. With cheap slm's, "c-weighted" comes the closest to being flat.

What are you feeding it with?

The filtered-noise signals most of us use for level matching start as RANDOM noise. Even on the extra-slow setting of my analog meter, which already is easier to read in a dynamic situation than a digital meter, the reading must be averaged.

Only a sinewave would give you a rock-steady reading, but they're highly subject to peaks and dips in response.
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That's what they do. You're going to have to average it out, and really........ listen, to ultimately dial it in. Nature of the beast. Be sure you have the meter on 'slow' response. That helps 'some'.

greg