Skyrim is supposed level you for every 10 skill increases, but sometimes, when I get a skill increases I get 2/10th of the way toward a level rather than 1/10th. As a result I'm leveling faster than I should (bad thing, because I have fewer skills for tougher mobs).

Tested it also using "player.incpcs " using new Nord character:
it takes 6x"player.incpcs heavyarmor" or 8x"player.incpcs illusion" to level, rather than needed 10 of either to level.

Can you help? Is there a console command that controls this? Is this supposed to happen?

... ruined my game!

Levelling slows down at higher levels.

Sorry but this is irrelevant: the speed with which you gain skills slows, yes.
My issue is that it takes less then 10 skill-ups to level once, which is a problem.

No. You need an increasing number of skill increases to level up as your level rises.

At low levels, this is not at all unusual. However, as JMC002 pointed out, it will require more skill increases to go up a level as your level increases. It is very difficult to stay low level in this game.

As it was pointed out before:

Leveling up a higher level skill will progress you to your level a lot faster than a lower level skill. You can even try it yourself, get to level 20 and then raise 2handed (or any other skill you haven't been raising at all), and see how little it contributes. Leveling up just sneak from 15 to 100 will get you quite a bit of levels, and those last few skill points (from 90 to 100) really contribute to your levels.