2006 CRF 250 R - Shim Issues

Ok, so I love my CRF 250 R. Hell of a nice bike and a nice kick to it, BUT, In 30 hours I've had issues with my intake/exhaust shims every 10 hours! At first I had no idea what caused the backfire and the kicking issues. Had it fixed, then 10 hours later, same deal. Was told an MX bike is not a good idea for dirt riding. . . . high revs, long distance etc. Was also told to use 95 Octane, our 93 Octane burns hot. So my last 10 hours was like this, SLOW! did about 40 - 50 km/h in 5th. The bike was cruising. After 5 hours had difficulty starting it, thought it was just cold. But after the last 5 hours, I can't kick this %$#%#$ bike to save my life, back to square one . . . . SHIMS. No compression, no start, backfire etc.

So the question is this? If I'm a novice rider (I don't race MX or dirt, but am average with my mates), how come I only get 10 hours riding between having to get new (alternate size) shims fitted???? This is freaking me out, after 10 hours of my riding the transmission oil is like new, I don't ride the bike hard!

The only other info I have, is that the previous owner skimmed the head. Not sure how much. Could this have an influence?

Would really appreciate some insight into all of this. Oh, one other thing, was told stainless steel valves could help out.

Thanx for the info.
Yip, I read the manual, thought servicing would be less since I'm not Villipoto. I do the basics like oils, filters etc, but not inside the engine. Suppose the bike still revs even tough I ride a bit slower.!

Will try the stainless valves.