Taco Magus, with all due respect, you need to make up your mind and stick with it.
It was easy for me to do that, because I intended to replay the game from the very start. I figured that if I wanted to try something different, I could do it the second time around.
There's something about character development that you need to keep in mind: you can't do justice to any of the classes if you only play through half the game with them (or less). Sometimes, even if you're frustrated with them, you need to give them a chance. It will pay off in the end.
If you have a multi-class character and you feel like he isn't advancing fast enough, then you can reduce your party size, even if it's just temporary. A Fighter or a Fighter/Thief can easily solo Chapter Three if you side with Bodhi. For the more difficult quests, you should recruit some help, even it's just temporary. For example, if I were "soloing" the game, I would take an extra Fighter with me to Nalia's Keep, and I would take a Mage with me to Windspear Hills. I've never taken the view that you HAVE to have any particular number of members in your party. You can tailor your party to fit the quest at hand. (Keep in mind that if you're involved in a romance, you don't ever want to dismiss your romantic partner.)
You could also try a dual-classed character instead of a multi-class character, such as a Berserker dualled to a Thief. You'll miss out on the Fighter's high level abilities in Throne of Bhaal, but your character will be very effective in combat and advance more rapidly as a Thief.
I highly recommend a multi-class Fighter/Thief, a Swashbuckler, or a Berkerker dualled to a Thief. The available NPCs can provide any other skills you need along the way.
If you want to play an Evil party, then the three Evil NPCs all make great party members. Just keep in mind that if you're not playing a Thief (or some combination), you might have to recruit Jan or Yoshimo (both Neutral characters) for your thieving needs. You could even drop off Jan or Yoshimo at Spellhold and let Imoen join your group. She's good enough as a Thief to handle most of your thieving needs, plus she's an effective Mage. If you play Throne of Bhaal, you should also consider recruiting the new NPC, who is an awesome warrior.
Ty to keep your party balanced in terms of skills. You need one or two "tanks", a thief (or some combination), a Mage (or some combination), and a Cleric (or some combination). Look at it this way. I've played the game (quite successfully) with a Cleric/Ranger and Imoen (a Thief dualled to a Mage) plus an extra Fighter I recruited for Throne of Bhaal. They had all the skills I needed. If your group has those same skills, divided among whatever party members you have, then you'll be able to handle any of the quests.