"Check Engine" light issues. Solutions?

Please, if anyone has experience with this problem, your insights (no pun interend) would be appreciated. Here's the prob: My "check engine" light came on and according the manual, the cause is likely to be a loose gas cap or some sort of emmissions problem. Well, it's NOT the gas cap. My mechanic said that the the computer indicated problem codes # 300, 301, 303 & 304, with 300 being a "random misfire" (?) or the like (my memory serves me poorly, sorry). So he after the tunes it up he wants to keep it to be sure he fixed it... but because I needed the car, I took it Weds eve and said I would bring it back on Friday. It's till "pausing" in idle cycle even though he speeded the idle (it has been "sputtering" lately).
And so then he calls me Thurs afternoon and asked me to bring it and leave overnight so he could check it COLD in am because it may be a valve issue (valves could be "too tight" or something). Does this seem plausable? Anyone else have similar issues with their 2001 CR-V? If so, what was the solution? Thanks! TheTorTor

Ok here is the story. Most people don't bother to get there valves adjusted every 30k like they are supposed to so what happens and only on 99-01 CR-V's is the valves get tight rather than loose and eventually it causes damage to the head. If there is a misfire code 301, 302, 303, 304 the best bet for your mechanic is when you drop it off over nite is to check the valve clearance (when its cold). If any of the valves are tight then he can loosen them and there may be a chance it could fix the problem. If it gets a little better but not a100% then i am sorry to say that you will need to put a new head on the motor.