Subaru 2009 Forester AT Oil Light and ABS light comes on, flashing while experiencing a jolting surge just prior

Two weeks ago, was driving 30 mph and my car made a kind of jolting
"surge" motion like I'd suddenly downshifted, but I didn't. Just afterward,
the AT Oil Temp and ABS light popped on and started flashing. Took the
car to the mechanic, they couldn't replicate the problem, said it was
probably just dirty transmission oil. Did a drain and fill and I was back on
the road until yesterday. Driving down the freeway at 65 MPH when
experienced the same 'downshift jolt' sensation, my RPMs revved up
high and the AT Oil Temp and ABS lights came on again. Mechanic, again,
can't seem to replicate the problem when they test drive. The light does
NOT stay on after starting up the vehicle again, so they can't pull a code.

Any ideas, guys? This car only has 58,000 miles on it and I am at my wits
end.