Priority System running too cold?

[Galliano]

Any advice for this one...........

I'm running a priority system on a 350 chev/turbine setup. All things going great for last 5 years, no temp issues, sits bang on 80 degrees. Last time out she sat at 60 and wouldnt get any higher. Replaced thermastat, checked hoses can't find anything issues, ran on Saturday and same issue. Only thing left is sender unit .......unless anyone else out there has any bright ideas?

Cheers

Galliano