727 Third Gear Issue

Figured I'd post on here before bringing my 72 Demon to a trans shop to see if anyone has any insight on what's happening.

The car has a 727 auto trans, 8 3/4" 489 rear end with 3.55 ratio and the steering column shifter was moved to the floor. All done around this year.
Recently, I've been noticing the car not shifting to 3rd gear. RPM's would go 4000 at about 45mph and would never shift to 3rd. I looked at my shifter and it looks to be on the Drive gear and not on 2. Does this sound like a transmission issue to anyone or possibly the shifter linkage? I forgot to mention that, in some days, I find it to shift to 3rd just fine also. So it's not constant. Not sure if temperature has anything to do with it or not. Any in put is appreciated. Thanks.