The HP roll on level up is rather annoying. Especially because it makes you want to reload again and again (not. for a mage which is squichy enough like that). Which is a waste of time, and not a great way to play. But there is no way your HP are lower after a level up than before...
You can install BG tweak http://www.gibberlings3.net/bg2tweaks
It has a lot of possible changes (some are nice, conveenient, or pure cheats, beware). During the install, you can pick each tweak you want to install, or pass.
But the nice one in this regard is :
Higher HP on Level Up
This component provides the player with a choice of two options that will increase average HPs when characters level up.
Maximum. This option will ensure that the dice roll will always be the highest available.
NWN-style. This option will ensure that the range of possible HPs gained will always be in the higher half of the range, i.e. results will always be 4-6 on 1d6, 5-8 for 1d8, etc.
So, or you get the max HP each time (which will make you really more powerful). Or you get rid of the lowest dice rolls, which is good enough for my taste. Really, it saved me from the temptation to reload for ages at each level up...