It may be the fabric you are trying to use. A lot of aged coated nylon can get gummy and gummy, sticky fabric can be nearly impossible to sew because it drags on the thread and needle. Try sewing some non-coated fabric (like an old sheet). If your machine can handle this then your trouble is likely with the old tent fabric. You may need to rethink the economy of repurposing an old tent.
May I walk in beauty.