As you know AD&D Characters gain fix HP's after level 10. But before level 10, CONSITUTION effects your extra HP per level.

There's 2 mages who advanced to level 9. Red robed mage's con is 16 (possibly a conjurer!) and Black robed has only 10 CON.

As they advanced level,

Red guy gains 1d4 +2 hp
Black guy gains 1d4 hp

So before advancing to a new level you can increase your total hp per level stat with girdle of consitution. I never tried potions but maybe they can work?

i dunno but the fixing for my fighter is pissing me off.

You can fix your problems with girdle of constitution at any time. Once your con goes back down you lose the hps so no there is no loophole... BUT the duration on the girdle is quite long. Give it to your character that needs it the most and use it before you go out fighting.