Yep! Fine one minute! The next I'm spitting out dust wondering what the hell happened and Skips is pratting about at the other end of the svhool!!
He even spooked at his own shadow the other day! It's ery frustrating cos he is a good dressage horse but you never know what he's going to do!!
I find that if you feel him tense up then try and breather deeply and relax and talk to him. Try patting/stroking his neck and reassuring him. If he is staring at something then maybe go into shoulder-in just as you pass it as usually this helps stop massive spooks and keeps them occupied. Also, if he is tense, do lots and lots to keep him busy. Don't give him a spare moment to think about spooking. If Skippy is tense he has a walk to loosen up etc. then a trot in his 'comfort zone'-20m circle at A in his case! And then we do loads of leg yielding and shoulder-in, spirals in off the track when we're circling and then back out etc. This loosens them up and gets them working nicely whilst keeping them busy! Stay in the 'comfort zone' until you're both relaxed and happy to go large (I once had a whole lesson in our 'comfort zone' because he had chucked me off 3 times in a row the previous ride and I was really nervous and was transmitting tension to him! At the end I was way more relaxed and he did some good work because I wasn't tense!
Also, if he is good with poles then set them up at 4 points of the circle so when you are circling he has to go over them and concentrate on them, this means he's not looking for things to spook at quite so much! You could also try setting them up in a 'fan' shape and go over them at different points of the 'fan' so one time he is shortening his stride and the next he is lengtheneing! It does really work to keep them occupied!
Basically, always be there, riding him. Never just sit there ad let him get on with it. Sharp horses tend to need to know you are there all the time. If he spooks to the inside then always have your outside rein in a good contact (this also helps with outline). Try to relax and breathe and if he tenses the don't tense up yourself. Just talk or even sing and pat him to let him know you're there!
Anyway, hope this helps! If I can help in any other way then feel free to PM me! My horse is a really, really, really sharp, quirky, spooky TB ex-racehorse so I know exactly what you are going through!