what is your idle rpm after warm up? You should be at about 650 rpm initially and not much higher when first going into gear. Assuming your idle is set correctly and there are no issues with your throttle control.... If you keep the outdrive in neutral and rev the engine, does the throttle control seem to increase and rev the engine smoothly with a nice paced increase in rpms, or does it jump? If you can rule out the throttle cable, then focus on the shift cable.
One cause could be that your shift cable is out of adjustment, causing its movement to be slightly behind that of the throttle. By the time that the shift cable moves the linkage on the outdrive, the rpms are too high and wham.
If you have the SX drive, try this - remove the back plastic cover to the outdrive revealing the shift linkage. Have a helper (with the engine off) move the throttle control in and out of fwd, neutral and reverse. Watch the movement of the shift linkage as the helper tells you they begin to move the throttle control. If the shift linkage doesn't immediately respond to the movement of the throttle control, you may have a bad cable, corrosion or a kink or you might just need to adjust the nut at the end of the cable where the cable meets the linkage to make the cable pull the linkage right away - essentially taking out "slack".
