Playtesting vs. Playing
This sounds like the GM is still attempting to tweak the design of the class you're playing. As a GM, I've often faced situations where a homebrew rule / class / spell / monster / thing didn't work out the way I'd hoped or envisioned it. If your class is too powerful relative to the others in your party, for example, s/he may feel a need to tone your PC down a notch.
It sounds like the GM isn't really communicating that well, though. I'd start by having an discussion about the class outside of the game session. This is pure metagame talk, so no need to make the other players sit around waiting during the session as you talk this out. For example, if you've overpowered your Linguistics check relative to what the GM had expected, he may be trying to balance against that.
I'm not saying that's the right (or wrong) decision, but I would start by exploring that motivation. Why isn't the class right? What would make it better for his definition of better? What would make it better for your definition of better.
Accept that this will require give and take from both of you.
Fix the issues
Negotiate to resolve the underlying issue he sees with the class, but in a non-confrontational way, and not in the middle of the session. Explain that constantly taking away the power you thought you had is really messing with your enjoyment of the game.
But accept that you may have to accept changes anyway, if you keep the homebrew class.
If things need to be changed, ask for warning before they happen. It isn't unreasonable to ask for him to give you a week or two between him redrawing the rules and the new rules going into affect.
Also, express your concern that you're being targeted unfairly. For example, if his beef is really that magic needs to be more chaotic, then express a desire to see that play out for all magic classes -- including NPCs and monsters. Not just you.
Ask for some give on his part, too. If he's intrinsically weakening your character, what's the trade-off? Maybe your character gains an additional spell slot per day or learns some additional true names / spells or something to help offset the higher chance of failure. Or maybe have the failure check roll on a fail table that has a 50-50 chance of failing in your benefit so the spell doesn't do what it should, but it somehow can still help you out in other ways. (AD&D 2e's Wild Mage rules had this.)
If the GM's rule changes are too drastic for you or he refuses to give anything back despite the weakening of your class, ask the GM if you can ret-con the PC into a different class. Maybe go with a stock wizard or sorcerer, but try to pick spells with similar look-and-feel to how you play the PC? (If you cast magic missile, but explain it as using a true name from an in-character-perspective, the result is close, right?) This is the "nuclear option," but may stop the conflict, at least.