bike intermittantly doesnt load when rolling on the throttle

[boldor]

Hi there , we have a 2001 GZ 250 with 12,000 miles on the clock. It starts well, idles well and is more than happy at 65 MPH on the flat

What it is occassionally doing , and it has been when warm , after rolling off the throttle and changing down, is when the throttle is rolled on there is nothing for say 1 to 2 seconds , then the bike kicks in and accellerates, till the next higher gear, and the same, nothing for say 1 or 2 seconds then away we go

This can last for a minute or so and then goes away

Of late it hasnt been happening, with a typical day being a 30 mile open road run to work each way, with some serious hill work included

any thoughts appreciated

cheers and thanks
leroy