Bicycle Tutor

Hi i have just bought a bike (saracen bullet 6061 alli frame) and have taken it out for the first time and i appear to be getting what id call pedal slip, when i pedal along without much force then its ok, when i exert downward pressure or change up to anything over 5th in middle range (21spd) this happens the bike isn't new but second hand and supposedly not ridden although i recon its had some use but not much, i have tinkered with the gears front and back as they weren't working through the whole range the chain could be a little slack but i'm unsure, the crank is raceface and gearing is shimano deore.

Also if anyone knows anything about this bike at all id appreciate it as i cant find any info on it at all...

thanks jon...

   
   

Initial thought would be worn cogs at the back, especially when exerting pressure?
Measure the chain as per this:-
http://bicycletutor.com/chain-wear/
If the chain is worn then possibly the cogs are as well.
The other possible is the freewheel ratchet slipping.
P.S. Just a couple of notes.
a) The saddle should not be tilted back as per the photo.
That angle may cause personel problems? It should be level.
b) If you put a drinks bottle in that rack then it's going to get covered in c##p or worse.
Get a bottle with a cover just in case, or find another place for it.

thanks 4 yr help