I just repaired a Sharp Carousel model R-209KK microwave oven with similar problems. It was very easy to fix.
I disconnected power and removed the top of the microwave oven. I loosened the touch panel (one screw was holding it on the inside - yours may be different). Several wires were running to a circuit board on the touch panel: I loosened only one wire (a green ground wire) - just enough so I could manipulate the touch panel.
The touch panel had two parts: a circuit board screwed to the back of the panel with 2 screws and the touchscreen itself built into the panel. I loosened the two screws and the panel (with touchscreen) came completely free of the circuit board.
I could now see what connects the circuit board and the touchscreen - a very fine and narrow strip connector. If you look at it with a magnifying glass you will see that it is a connector of some sort: it presses into a corresponding connector on the circuit board. This proved to be where my problem lay: the connection (which relies on pressure and position) had worn loose or dirty over the years possibly due to relentless pressing and punching of the panel buttons and the door opening button.
I did find a small smudge of material on the connector and removed it with my fingernail. I replaced the circuit board, being careful to position it properly, and tightened the screws. I reconnected the green ground wire and tested the unit. It worked!
I suspect that in time this connection shrinks or is jarred, or comes loose and the alignment is lost. Then the very fine connections in it fail randomly leading to unpredictable touchscreen behavior. The solution is to take it apart, check for dirt and put it together again being certain that it is aligned and tight (all done by ensuring the two screws are tight).
Michael Kersey
August 2008