Bike Forums - Freewheel, crank, or chain?

Im having a problem with my bike where I cannot tighten my chain correctly. Here is a description of the problem. When my crank turns to a certain position, my chain is extremly tight. If you pedal it about half a turn, the chain becomes loose and likes to slip off on me. The rear wheel is screwed in tight so its not moving and when I try to adjust the wheel so that the chain does not go slack on me, right at the part where it goes tight the crank will not move and therefore I cannot pedal. Info about my bike. I have a mongoose hoop d frame. Not those new ones that I might add are badly built. An older chrome one that looks exactly like a royce union. It has an Xgames three piece crank and a next two piece sprocket on the crank. The back rim is also an Xgames rim and has a small, very small, bend in it. If you pedal the crank backwards while riding, the freewheel will make a high pitched sound and move down to a low pitched sound. I was wondering if anyone had heard anything about this and if so, if there is any possible way to fix it as money is very tight for me at the moment. Plus I have looked around for an Xgames freewheel and am having difficulty finding one. Any info would be greatly appreciated. Thanks:)