178lbs is good at 5'6" (more-so if he was leaner)...it's not that far off what you could expect for a natty.
I do agree on the bulking/cutting cycles though; if bodyfat is constantly reaching 18% often, likely you need to go easier on the bulking calorie intake. If you have a more "endomorphic" body type, you probably need to aim for less gains in bodyweight monthly.
Or at least, you may need to trim up for longer (i.e. don't be the type who looses 5-10lbs and gets scared from the "flatness" and bulks up again). Don't kid yourself with how much lean mass you have - if the scales go down more than you expected, but you are maintaining strength, it's not likely muscle you are losing lol.