OK guys, enough horsing around. I have done precisely whatyou want. I have a frequency meter that measures the horizontal and vertical frequencies and pulse widths for computer video systems. I measure the vertical frequency to 0.01Hz and the horizontal frequency to 0.01KHz. The results are displayed on a 128 x 64 graphic LCD.
For those of you who like gadgets, I'll sell you one for $295.
The basic method is to measure the periods and calculate the frequencies. I use one timer in edge-triggered mode for period and another in gated mode for width. It works very well.