Glenn, Yes, from the FSM on DTC24 it looks like it could be the ECM or the chip ( MEMCAL or EPROM ). Do you have a stock EPROM that you could swap in just for a test? Maybe someone near you has a spare ECM ( known good ECM ) that they can lend you to just see what happens.
Also, on the shift to 3rd being smoother.....I'm a bit concerned for your clutch hydraulic system's performance. How old are the master & slave? What is the condition of the fluid? Usually balky shifts are a result of system degradation...usually. Check out the link to zfdoc.
http://zfdoc.com/ there is a good Q & A on diagnostics over at Bill's site.
Tom