too slack?
i have a similiar problem on one of my bikes
on my giant ocr-3, my mechanic replaced the entire drive train
its a road bike, but he gave me a mtb cassette (its bigger)
subsequently, i think the chain maybe too big now, to accomadete the larger cogs, when i use the smaller ones, with the smaller crank, after i pedal fAST, i notice a "hick-up" in the chain
the hick-up is not so bad, and i noticed it mosty when pedalling fast and stop abruptly.
i don't know what kind of bike you have, but if its an older one, the rear wheel may have a larger "shaft" then the ones nowadays, where the wheel fits snug, if so make sure the wheel, and chain is snug.
also, i recommend finding a good mechanic, your bike may be unsafe, and seek a pros help
your almost better off collecting less bikes, as you accomade more, the expense increases exponentionally to maintain them all, self-edification is great, but some times a little knowledge can be a dangerous thing