Is it on all bands? If so, the wires might be a bit slack.
If it's on one band only, could it be an intermittent shorting link on the folded dipole twin lead? That would make that sort of magnitude change in the VSWR.
"In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be." (Lord Kelvin 1824-1907)