I assume, but am not sure, that this is caused by someone else in the house competing with the shower for cold water.
I'm guessing an anti-scald valve might be necessary but perhaps there is a way to keep the cold water pressure higher so that it doesn't drop as much when someone flushes a toilet or something. The shower in question is on the 3rd floor.
Added:
I have since learned of at least three approaches.
1) pressure balanced valve
2) thermostatic mixing
3) anti-scald shower head
Here is a good site comparing the first two: http://www.hometips.com/buying-guides/shower-valve-anti-scald.html
The anti-scald showerheads I have found online are very inexpensive (under $30) but poorly reviewed.
Best Answer
Adjusting the whole-house water pressure is unlikely to solve this problem, simply because you set your shower temperature based on some initial balance of hot and cold. When other people draw cold water only, the pressure difference will still have the potential to affect your shower temperature.
Instead, you should get a pressure balanced valve or shower fixture. These fixtures automatically reduce hot water pressure when cold pressure falls, so instead of a temperature spike you simply get a momentary decrease in output pressure. You can find pressure-balanced fixtures (which would replace your existing shower fixtures) or an independent pressure balancing valve (which you'd want to install near the shower but would allow you to keep your existing fixture).