Sluggish, hard to shift gears

I recently made a 160(round trip) mile run , the following day my C50 blvd felt sluggish in first and second gear. I really didn't think to much of it and continued on my trip just a few blocks from my house. All the other gears felt okay  and shifting was smooth.  This Saturday  I made another run 380 miles(round trip)to a camp site to meet up with some friends. As I started this run again my bike felt sluggish and gears were hard to shift, I did manage to make it to where we where headed without any problems. On the return trip my bike was again sluggish, gears were hard to shift and it died out on me a couple of times before we got going. After a few miles again the gears were shifting without any problems and the bike returned to running really great. Didn't experience any other problem with bike and made the return trip home alright. can anybody tell me if they have experience something similar  and how you fix-it or what was checked? I'm planning a trip to New Mexico in a couple of weeks and don't want to risk getting stranded anywhere. Please provide feed-back.  

If you have some problem with your bike like shifting a gear problem then you should check it from a mechanic and pick it up at the service station.  
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