its electrical. my tach does it as well. the connectors are at the bottom of that housing and can take moisture or rain (and ya with a convertible, it happens) and gunk those up. im looking to probably deal with it when i remove the dash to install a fresh heater-fan. a good wire-brushing of every electrical contact, some CRC cleaner spray and dielectric grease of each loom. its kind of fun and it gets you outside and its pretty hard to screw up. a nice way to get to safely know the ins and outs of your new spider.
my spider was garage kept its entire life and all rubber and hoses are tight and in great condition for 17 years now, and i have the sometimes-dead main gauge issue. i really dont think it should put you off of a good find, imo. its a minor detail