toughees

Hi Can anyone assist me with this. I recently purchased a new road bike. I have ridden about 200km on it. On every ride teh gears tend to slip and also lose tension on the crank. (I dont knwo if the 2 are related). I am a newbie at this so please excuse me if i am not relating this precisely. The LBS says that I should use the big cog with smaller gears at teh back, and use teh small cogs with teh larger gears. Pelase Please heklp riding the 94.7 and dont know whether I should take teh bike back to the LBS. Regards

Any advice will be appreciated. I have tried doing 2 long rides now(85 to 95km), but on both occasions I have bombed out around the 70km mark. Can anyone give me suggestions please. I am trying to just complete the 94.7 (before being cut off) this year.