S-10 stumbles and stalls

My 2003 S-10 has acouple of issues. 97000 miles. Recently tuned up. Plugs ,wires, cap, rotor,fuel filter. At most speeds, but especially at highway speed, it will stumble along. Sometimes cutting out and then resuming. Sometimes stalls when stopped, but always restarts. Seems to have a vibration almost as if the transmission is slipping, or torque converter is locking.

My mechanic has checked the fuel pressure, and all is fine. No codes in computer. He has only experienced the issue of stumbling, not the stalling.

Any ideas appreciated.