I think I'm having this issue too.
2001 Focus - SOHC SPI 2.0L. 192k km.
Car revs fine from 1->2 and 2-3. RPM will go from 700ish at idle to 2200-3000ish (conservative vs leadfoot) but going from 3-4 it seems as if I can't get it past 2400-2500rpm without putting the gas all the way to the floor.
If I back off the throttle, it drops in RPM to around 1600ish, so I'm not sure if that's an actual shift up to 4rd, or just losing all that power and going back to baseline 3rd.
I have had no CEL/MIL light up yet regarding this.
I'm going to be doing a trans flush/fluid/filter change next weekend (haven't had a filter change yet, just the fluid) so I suppose if it can be rectified by a replacement solenoid, I may as well replace it while I'm in there.
Can anyone shed some light on this for me?