derailer problems

so i guess this is the best place for this post

i have a bike im building today took it for a ride the rear derailer was sliping 2 times every rotation but only on 5 to 8 the low gears were just fine

heres the setup
a new chain
a 8 speed cassette
a three speed crank
and a 8 speed rapid fire ****fer i should also mention i put this bike together from just a frame so i cant just put the old part back on ohh and everything is emaculantly clean i and seams to be functio9ng perfectly the deraler is smooth the gears on cassete are like new and the chain is new the crank is great shape also but im sure the problem is in the derailer or chain size or whatever

i just dont understand when i put it on a bike stand and ran it thru the rear gears it shifted up and down thru each gear with out any problems clicking or sticking but as soon as i pedal i get slipping or something even with the littlest amount of pedal pressure slipiing im not sure if its trying to dersil up or down

this is what im thinkng maybe the derailer and shifter arent lining up properly or maybe the shifter and cassete are off or possibly the chain is wrong size for rear cassete

i am stumped i hope someone has some input i really want to put a jackshaft setup on this bike but with bad shifting i cant

yeah see i dont know if the shifter and derailer are compatable but i dont see why not the derailer simply moves from tension maybe the shifter and cassette are not compatable or it may be a wrong chain i dont know how to check ill go to the bike shop later in the week and pick some brains probly start with a new chain since i have a old bike that could use a nice new chain

so i pretty much built this bike from used parts for virtually nothing but it took hours of digging thru stacks of used parts looking for nice components the only new parts on this bike are all new bearings in head set botton bracket and hubs

well i guess ill go back to the co-op and digg around and try different shifters and see if that works i really like the current design very smooth out of the way and in great shape thats why i chose it

if i have some cash ill try a 8 speed next

the cogs on the cassette look great so if a chain and shifter dont work ill try a cassette

im sure the derailer is fine but if everything else dont work that will be the last thing to try

ill let you guys know what works and what doesnt

heres a few pics of the bike it really rides nice and smooth in the gears that work and the brakes are awesome easy rear skids