I beg to differ. MIUI is my daily driver and I don't get any random reboots; none. I'm currently running 0.12.3.2 as It's been rock solid stable for me and neither 0.12.11 nor 0.12.18 seem to add anything new and exciting for me, nor do they seem to fix any small bugs like SMS time stamps.
If you're getting random reboots, I recommend you try a clean install again, clean install apps directly from the market and take a real good look at the apps you want to install, try to keep it to the bare essentials first (my gut feeling tells me certain apps a lot of you people use cause more harm than good on this and many other ROMs).
I also recommend getting Startup Manager and disabling every non essential app at startup (startup manager is the only app I've ever found to work 100% as it uses root to disable apps at boot). Apps like foursquare, gowalla, paypal, loopt, qik, documents to go, my verizon and others are really unnecessary at boot time and just unnecessarily tax the system.
If you wish to flash an alternate kernel, make sure to choose an appropriate kernel, keep in mind that no one needs anything over 1Ghz, seriously. 800Mhz - 1Ghz should be optimum and more than enough. Also try different voltages if you encounter any problems.