Sounds like you have an idle problem related to an intake leak or stuffed idle control valve or an AFM problem. Give the AFM a clean and check for leaks.
When the RPMs drop real low, like 400 and below, the oil pressure skydives and im not suprised if the gauge almost fell to zero. This is becaues the crank is spinning very slowly, so the oil pump is not pumping fast enough to replace the oil loss at the bearings hence, very low pressure indeed.
Btw you've checked your oil level yeah? (The oil light coming up during braking on the other hand is not a good sign ie next to no oil in the sump!... unless you have an auto and the revs dropped very low during this time and fits the explanation above)