Are you referring to the wheels wobbling around making the steering unstable or an engine misfire? For handling problems have it inspected at a tire and alignment shop. They'll look at things like ball joints, tie rod ends, and control arm bushings.

For a misfire, you need fuel, spark, and compression. Missing any one of those will cause a misfire. Neglected maintenance can result in worn and misfiring spark plugs or deteriorated spark plug wires. Low fuel pressure and plugged injectors will result in a misfire from lack of fuel. Loss of compression is always due to serious internal engine damage but that will never be intermittent. You provided so many details that we have to guess at the symptoms and observations. If the misfire ever clears up, it's not caused by a compression problem or other mechanical engine problem.