My recently rebuilt 87 FXRT loses most oil pressure after 2 miles or so. Starts fine, runs quiet for a few minutes, then loses pressure down to about 8-10 psi and the valves get noisy. Runs very well and quiet on first start up. Seems to be heat related, either from expansion, or oil thinning. Nobody has answers. I´ve put about 80 miles on it since overhaul. Any ideas?