2009 MINI Cooper clubmaan misfiring

Hi,

I had a question about a MINI we recently purchased.  We just bout a used
2009 MINI Cooper Clubman (automatic) with only 45,000 miles on it.  It had
a full tank of gas when we bought it.  We accidentally have filled it twice
with 87 octane gas (not realizing it was to use a higher octane).  I then took
it on a trip to Pennsylvania last weekend (about 3 hours away).  I noticed it
rough idling once I got there when stooping at stop signs, lights etc.  When
driving home I noticed the same, and now have had the check engine light
come on.  It does not seem to do it when I first start it but after running for
awhile.  I now have had it stall when sitting at stops, and have had the
check engine light come on with codes of "Random Misfire" on multiple
cylinders.  Could this be related to the gas?  I got down to about 1/2 a tank
left of the 87 octane, and at that point filled it with high octane, hoping that
would flush it out, but have only gone about 150 miles since then.  I am not
sure if I should try to run out that tank, or if I need to get it checked out.