94 toyota pickup speedo

I have a 94 Toyota pickup 2wd with 3vz & 5 speed. Speedo is intermittant. Changed sender and cleaned the terminal at sender, and packed it with dielectric grease. Worked ok for a while. Seems to be worst in wet or cool weather- Hawaii, it doesn’t get cold. How does one test the meter? I’d think they are rather pricey. Nice truck, very low mileage, worth the fix but don’t want to spend much money right now.

Ideas?

not a reply but additional question- Is there a way to adjust the mileage? it is an electronic speedo. New or used, the mileage will be wrong