HealthBoards - Orthostatic Confusion!

Hi everyone, am hoping someone can offer some advice... I'm 45 and have a couple of health problems - ME, IBS and Graves' Disease (under control). When I stand up my BP (sometimes) drops, sometimes it rises quite a bit and my pulse rate also goes a bit wild. I've had a few falls over the year due to dizziness and I have trouble standing for long - in supermarket queues for example - I get palpitaions, lightheadedness, nausea and just generally feeling like I'm gonna pass out.
I've been put on a daily BP monitor that takes three readings - lying down after five mins, immediately standing up and standing after three minutes.
I would've said Orthostatic Intolerance but the readings seem pretty normal to me...Don't panic; I'm not gonna list 'em all ;) but these are pretty indicative:

Lying 117/78 (Pulse74), Standing 118/101 (102), 3 mins 123/102 (90)
L 105/68 (76), S 105/86 (103), 3 mins 124/82 (86)
L 109/76 (80), S 98/88 (107), 3mins 108/90 (96)
L 112/72 (75), S 90/78 (94), 3 mins 111/86 (76)
L 117/72 (74), S 94/85 (104), 3 mins 116/85 (85)

Does anyone else have anything similar? Don't get me wrong, am not stressing, but would like to understand what's causing the symptoms. Thanks! :)