'94 Caravan 3.3 MIL

My son's '94 Caravan 3.3 has a recurring MIL display while diving. After shutting vehicle off it resets itself and then comes on again
after a few minutes, usually on highway driving. Cycled the key to read codes, it displayed the following (in this order): 12 Memory standby power recently lost 32 EGR diagnostics 31 Purge solenoid circuit 55 End of test The vehicle seems to run and drive perfectly normal, even when the MIL is lit, which led me to believe from the beginning that the issue is predominately an emissions issue. The battery hasn't been disconnected recently. Aanyone know of a common problem with this year and engine to look for first? TIA
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