I have this problem with a horse I train. She is a 19 year old OTTB with the mindset of a horse fresh off the track. With her, I do a lot of work refocusing her attention.
First: I work with a lot of loose rein. This might sound super counterproductive but if you relax her into a long frame then you can get her to relax into any gait. I do this by getting into a half seat at the walk; I think that when she feels the extra weight (im heavier then her owner) she freaks so the half seat helps relax. Then I just really work on getting her head low with a light contact on both the inside and the out and slightly playing with her bit. The second she offers to slow at all I reward her with more rein and a light scratch on the neck.
Second: this may not work for you but I think that she works really well with a solid rein contact and leg yields. The rein helps to stop the forward motion and the light leg pressure on the inside/outside makes her use her body in a productive way. She is MUCH more tired after 20 minutes of leg yields then after the same amout of time spent fighting my hand and cues.
Third: the moment she starts even speeding up within a gait I switch whatever I was doing. If I was in a straight I bend, if I was in a yield to the left we go to the right. What ever it was that was making her tense up enough to want to jig/take off, we change.