I know the 'go to' answer for this is the head gasket. But: the mechanic just did an engine overhaul and same problem persists.
Allow me to explain. In November 2018 mechanic did work and replaced transmission. Within two days a coolant hose blows on the car. Car overheats but not significantly. Hose gets replaced. Notice low engine Horse Power on acceleration, and transmission hard shifts to 2nd. Brought car to mechanic for check head gasket (possible low Horse Power issue) and evaluate transmission. Mechanic diagnosis as low idle and ignition tune up needed. Replaces idle control valve and ignition components (distributor and cap, rotor, wires, plugs. Says when asked that he did check for head gasket. Pick car up, and same problem persists. I check for exhaust in coolant and find exhaust in coolant. I bring car back and proceed to rip the mechanic a new butt hole. They rebuild engine and machine cylinder head and engine block. Machine shop says block and head not cracked. I pick car up today. Same problem persists. Pressurized exhaust in the coolant.
Is is it possible that there is an exhaust cross connection of some type related to the installation of the transmission or idle control valve? Any thoughts would be appreciated. Going crazy in Cali.