1988 Lesabre, with vehicle at operating temp I am getting a shudder at around 50 mph. this problem occurs in overdrive and third gear, if I lightly apply the brakes while not backing off the gas the RPMs increase and shudder goes away, as soon as i let off the brake it returns. Trans fluid is full and I've added sea Foam .I've been through all ignition components ( coil and module, ecm, wires, plugs. O2 sensor, egr and more) . Thanks for any and all help.

Give the additive a chance to work.
When was the last time the fluid and filter was changed?