tdi 110 bhp acceleration stops at 20 and 50 mph

[audi_rs2]

hi
I have an audi a4 1998 1.9 TDI 110 bhp.
I have a little problem and I do not know what to do.
sometime in the second speed when speed is no longer raise more than 20 miles and sometimes in a 5 speed at 60 mph.
I think that is limmode????.
starts every time the first key and go normal without fault, but sometimes is got these symptoms.
has anyone any ideas what happen.

thanks