What am i doing wrong?

[jr_tech]

It sounds to me as if it is loosing B+ voltage... as the B+ voltage drops the vertical oscillator quits, leaving a horizontal line... then the horizontal oscillator quits (which will indeed kill the high voltage) but a dot in the center may remain for several seconds until the high voltage filter capacitor is discharged, then fade out slowly. Does this describe what you are seeing?
If do not have a multimeter, get one... the $20 one linked above is a killer deal! With that you can monitor voltages in the set as the problem occurs.
If the set has a tube B+ rectifier like a 5U4, you might observe it as the problem occurs perhaps the filaments go dark or the plates turn cherry red or the whole tube glows purple. Might be a clue there!
jr