I downloaded font “u8g_font_gdb30r” to userfont position 200 of my monochrome digole display. 

I then wrote a tiny program with a loop counter variable and a delay of 100 milliseconds to simulate a clock counting in tenths of a second increments.  Using font 200, while only one digit is displayed, the count looks good.  When the second digit is displayed (>9), the numbers start to skip, ie. 10, 13, 15, etc.  When the third digit is displayed (>99), the skipping increases to 110, 121, 130, etc.

Changing the font to 120 yields a good, non-skipping count.  Changing back to font 200 and the skipping is back.

Any ideas how to solve this problem?

I found my problem...

If the uploaded font exceeds 3584 bytes then things get weird.  I just tried a font that is only 3001 bytes to userfont 200 and now things behave much better.