Firstly. Nothing to do with the prop tips!
Put very simply.
All aircraft have an optimum cruising flight level for best speed. It does not follow that the higher you go the faster you go.
Alex is right with regards to EAS, so I won't repeat that.
Your Aircraft's optimum cruising level for greatest speed is FL180 depending on variations in temperature. It is the point when the mmo begins to decrease. If you continued to climb your barbers pole would begin to decrease resulting in a reduction in IAS and therefore a reduction in TAS.
If the mmo stayed the same your aircraft would exceed crit mach and I'm sure you have seen the TV footage of aircraft that exceeded crit mach!!!
I am not familiar with the Q400 but my guess is it's wing has very little sweep and that is why your IAS penalty for an increase in FL is considerable. A jet with alot of sweep would not suffer such a penalty because the pressure exerted on the Aircraft's wings would be less.
So, if your late FL180. If you've got limited fuel climb!!!