Oh    that sounds familiar, to make sure that your hot water cylinder is not the problem check the other taps and shower if it doesn't happen there then its not your hot water cylinder! The most common reason for this happing is a washer, when you run hot water the washer expands and your water pressure reduces. So check your Tap washers (replace if necessary) or if you have a single lever check the mixer assembly (mite need a new assembly), before you do that shut your water off at the meter. Also another reason for this happening is grit in the mixer. In this case just remove the mixer assembly and clean it out.