Fits in sleep

My Father, age 62 yrs. has fits in his sleep, nearly twice a year on average, they consist of his body trembeling, stiffening and convolusions, he breaths heavy & doesn't seem to be capable of open his mouth wide adequate to breath effortlessly, during this time he doesn't respond to anything, talk, comforting or pain. These concluding about 15mins then he calm down, still non responsive.His body un-stiffens, and he tries to raise himself out of bed, and whimpers as if he is frightened - this closing on average about 5 mins, then falls stern to 'sleep'. He remember nothing of and lately they've become more violent, and he's completed up biting a very small portion of his tongue . We have be to Dr.'s they see no signs of Epilepsy and cannot tell us what it is? Has anyone ever encountered such fits?
There is no pattern to it any. His test says normal.B.P. normal.
Any assist or information of what could be the cause would be extremely helpful.

Regards,
Sheetal.