Trying to figure out 3 year old's night spasms

Our son has had intermittent phases of spasms at night that wake him up and impede his sleep for an hour or more (once even 3 hours). He had quite a few last fall, and we thought, initially, that they might be seizures. He often makes rigid hand gestures (bilateral usually, but not always), has leg rigidity and movement, and sometimes makes weird smiles/grimaces on his face. He seems to be sort of in a reverie, but then when we talk to him, he eventually (but not usually immediately) responds. We've had an EEG and a sleep study -- he has mild sleep apnea, but no evidence of seizures (fortunately). They'd gone away for a while, but the last couple of weeks, I've noticed some mild symptoms. We had been treating him with iron supplement, since the doctor suggested paralytic limb movement disorder (PLMD) as the diagnosis. Last night, he sat up, his head was tilted at a very strange angle, and he was shivering. His temperature was low, even though the room wasn't especially cold, and his heart was beating fast (it usually does during these events). I asked him if he was ok (he's now almost 4, so he's a little better at communicating than he was last fall), he hid his face under the blankets. He said he was scared (but I think I asked him if he was scared, so...) and that his stomach hurt. 

I've never seen these behaviors/movements in anyone I've slept next to (my husband and I sleep next to him, alternate nights, because he wakes up scared if he's alone...something we'd like to change but haven't yet). Are these night terrors? They seem so deeply physiological that it's hard to interpret what's going on, even though he's had a neurologic eval and a sleep study. Our pediatrician doesn't seem concerned (when we dealt with this in the fall/winter, but I haven't re-approached her yet about it), but it's scary and also bad for everyone's sleep. I also don't want to subject him to endless tests if this is something that is within the realm of "normal."