Hi there

Im not a TV technician/engineer - so I can only guess.  It could be a loose/faulty LDVS cable, or a faulty T-Con board.  I doubt it would be the panel itself, as you mentioned tapping the back of the TV would work.

However to be absolutely frank - that TV model is from 2010.  TV Technology has come a long way since then.  And if you have a look around at the cost of a new 32" TV (which you have already purchased), then its really is un-economical to repair.  If you were thinking of repairing it and then selling it on E-Bay, Gumtree etc, really no one is going to purchase a TV these days that is 6 years old.

Anyway, just my 2p

Cheers