1996 Volvo 850, mileage 155K, ATF recently flushed. I recently noticed that lock-up hasn't been engaging consistently after A/T shifts into 4th gear. That little tiny final "shift" is missing, and RPM's hover higher than normal when cruising down highway and accelerating. Lock-up only seems to work for the first 10-15 minutes of highway driving when it usually disengages and won't engage again until the car is being driven cold. The A/T light has not come on.

Is this fixable without removing or replacing the tranny? I think I read there was a solenoid that controlled the torque converter lock-up? NOTE: I can't tell if lock-up works in 2nd & 3rd gear, not as obvious as in 4th gear.

Any help would be appreciated! Thanks...