2003 Honda CR250 help

I have owned my 250 since close to new and have really had no issues with it until now. I am pretty mechanically inclined and this issue has me stumped. So my bike started to act different when I was in the middle of a race. It had no low end power at all. So I cleaned my carb didn't fix the problem. Checked my air filter (clean every time I ride) and put a new one on, didn't fix the problem. Replaced the reeds, still no help. I rebuilt the top end of the bike before the season start, at this point I had 8 hours on the bike and it ran perfectly fine before this issue. I ended up noticing that my power valve was not wanting to close while in the lower end, well it did intermittently. So I but a new top end in and made sure my powervalves were spotless, still didn't fix the problem. Bought new power valve cables and had them properly adjusted by a Honda Dealership (Larsons cycle). They claimed it was fixed, well it wasn't. Lastly I replaced the power valve motor and still nothing. I was wondering if anyone has ever had this problem and/or might have any ideas of how to fix it. Thanks.