Logic. Just a bit of applied logic.
First, what you are seeing is a snipe. Not like chasing one in the woods, but a snipe - fore or tail. You run moldings, so you have seen snipes before on moldings, usually tail snipes. Planers will also snipe, fore and or tail.
Second - determine for certain what you are seeing on your doors - fore snipe is the first leading edge of the panel. Tail snipe is the last.
Now, for logic. Measure how long the snipe is. Not how wide, but how long. Lets say 5" tail snipe. Now go to the machine and open it up (power off, 100%) and visualize your panel feeding thru the machine. Take a thin stick, mark 5" on it and 'feed' it thru. Where the 5" mark is on the abrasive wheel (the sanding cylinder), you will see the end of the stick coming out from under the holdown roller. This allows the tail of the panel to rise into the abrasive.
Careful observation will tell you what is happening. Careful analysis of these events will help tell you what to do.
It ain't simple, but it is a good exercise in logical thinking. The age of the machine would lead me towards a bearing problem/solution, but that is not a given.