On my bike- the nuts that hold the sprocket on weren't torqued down enough, and the sprocket was able to shift on the 4 bolts about a 32nd of an inch.
That was just enough to throw the chain from perfect slack to crunchingly tight with every revolution.
fixed my friends bike like this too-
Loosen the sprocket just enough so it can slip back into place when you squeeze the chain together.
Spin the wheel and squeeze the chain when it gets tight until it doesn't get tight anymore.
Tighten the 4 lug nuts on the sprocket to the proper spec, adjust the chain tensioners, and VIOLA!
That isn't the problem on all bikes, but that fix worked for both of us.