I had the same problem with my new F200, got it in march. I took it apart and put it together again and after a new lcd calibration everything worked perfectly. Must have been a badly mounted cable between the lcd and the motherboard or something like that causing this.
A little bit to slow i think.
Mine runs at 3.5MHz, i suppose 48khz should be 48KB RAM.
Anyway it is a nice little computer but with a bad keyboard, but not as bad as the one on the ZX81.