I had a similar issue with our C30. Fixed it by tieing one end of thin bungie chord to the stainless pedestal mount and looping the other end around the throttle. I removed the throttle loop end when we left the boat so it wouldn't take on a pernanent stretch.  It was a bandaid solution, but worked for the 15 years of our ownership.
It would seem that there needs to be more friction between the internal cable and it's external plastic sheath. Perhaps the plastic expands slightly with age and relieves the friction. Maybe you could experiment with tightening some very small hose clamps around the plastic sheath. 

Remove the compass, reach into the binnacle and adjust the friction screw.

We also had that issue in our Catalina 30. I added a stainless steel u-clamp to the plastic cable jacket and gradually tightened the two nuts until the throttle held in desired position. That worked for me and I never had to adjust the clamp.

Airbus
2002 Catalina 36 Mark ll 
Hull 2074
Tall/Fin

Hi Kids,

We had this problem too. After replacing the shift and throttle cables we found that they moved too easily and would not stay "put". Haro suggested the correct and proper fix. Underneath the Compass, inside the binnacle resides the friction adjustment for the throttle and shifter cables.

Cheers,
PO
 

We'd never know about all these nuances of our boats if we didn't have this website.

We created that same problem for ourselves when we thought it would be a good idea to lubricate the throttle cables with T-9 Boshield Lubricant . Not really a good idea ! Eventually, we went with the cable clamp idea since it meant removing the the Navpod, then the compass light and bell just to access the friction screw inside the binicle. Way to much effort to adjust a screw. Use the cable clamp or the velcro solution.

The correct way to address your issue is as stated: remove the compass and the weather shield between it and the top/interior of the binnacle and tighten the bolt on top of the shaft that the throttle handle goes through.  This is a  fairly simple procedure. I did an article for the Main Sheet two editions ago on replacing the helm wheel brake and did a step by step discussion to get to this area. Depending on the difficulty of removing the 4 long bolts that attach the compass bracket and the weather shield to the binnacle, your project should take just a few hours start to finish.