30mph judder

[quest63]

This may be a strange one?

I have for some months been getting a judder at 30 mph cruising in 4th gear.

Previous to this I felt when doing this speed in 4th the car was slightly accelerating, or if not accelerating keeping going if I came off the gas pedal.

Sometime just below 30 in 4th its not juddering and then just above 30 its not juddering but most of the time just below or just on or just above 30 it judders.

Mileage doesn't seem to be affected.

Whats the likely cause guys?

Thanks in advance