I am hoping to get some input on a problem I have been having that my dealer has not been able to work out.

My 1998 XC will "buck" (for lack of a better description) at speeds between 50 and 75 mph. It feels almost like an engine missfire, it is very quick, the car "bucks". It is very noticable on smooth road surface, slightly uphill grade. No codes come up on the diagnostics. My fuel mileage is good, around 24-25 highway. It is intermittent and I am frustrated because the mechanic never feels it when I have it in. One note, at idle, it will occasionally shudder and the rpms drop and then return to normal. Again, this is intermittnet and infrequent. I just bought this 7 months ago with 17,000 mi on it and now have 38,000 on it and worry that it is a symptom of a bigger problem. Has anyone else had this problem? Any thoughts would be greatly appreciated.