Stumbling 35-50 mph while cruising?

[emerson]

I have a '97 with the 5 speed auto. No cel, when I downshift or accellerate hard the stumbling goes away. Did intake gaskets last year, O2 sensors, new exhaust manifold back, fuel pump+filter, water pump+rad+thermostat, it still stumbles intermittently. I've got 205/75/R15 tires on so it's at a lower rpm than stock at the same speed; don't know if that's an issue. It's like the tq converter can't decide to stay locked or not...maybe. Any thoughts? 240k kilometers on the clock.

 

[BrooklynBay]

Check for vacuum leaks, clean the MAF sensor, and check the DPFE sensor.

 

[emerson]

Cel on today. Code was P1445 evap surge canister..... Also thermostat is leaking.