Another thing it could be is the switch on the clutch. If you depress the clutch with your hand you can see a plunger type switch. Turning it 1/4 turn will release it. Sometimes the contacts get cruddy. I've tried to clean it, but just ended up buying a new one.
https://www.ecstuning.com/Porsche-1998-Boxster-986-Base-Convertible-H6_2.5L/Braking/Electrical/ Any of these will work as the brake light and clutch switch work the same. But if you want the actual clutch switch.
https://www.ecstuning.com/Search/SiteSearch/99661311401/ES1494411/ This is the actual switch. I'm sure with the part # you can find it cheaper on ebay. Keep in mind if the either the brake light switch or the clutch switch is loose in its mounting it could cause intermittent cruise control issues. On my car the clutch switch was a little loose, I just turned the switch a little till it was tight in the mounting. I hope this rambling post was of some help.
1998 Porsche 986
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