OMG zip, you really crack me up!!!
I haven't had this much fun reading serious forums such as this in a long while...
Anyhow, to echo what zip has mentioned, beta blockers have this known side effect of really slowing the heart down (and thus reducing BP as a side effect) which in some people may manifest in a lightheaded feeling when you exercise and the heart is not pumping fast enough to supply enough oxygen to the brain. I've experienced this at times too, although my dosage is really really low.
Regarding the BP, I would imagine it would be tremendously difficult to take it while you're exercising as described by zip. I've never taken mine while exercising and don't plan to, as we all know that BP goes up during exercise, although by how much, I can't really say.
Considering you have sinus tachycardia, how fast or slow does your heart normally beat on the low dosage of the atenolol? Have you measured it while exercising?