Engine hits wall at 5k only in 3rd gear

Strange issue came up this year. 340 is running great but when I get on it to 6k in first and second I then go to 3rd still at full throttle but at 5k rpm it falls flat. I noticed this once last fall but it went away. Anyways any thoughts?
69 340
4 speed w/ 3.23 gears
Lunati 60403 with Hughes springs at proper spec
273 rockers
LD340 w/ 850 thermoquad
Older Points distributor with the pertronix electric conversion kit.
Stock fuel line 5/16" with electric pump

Strange that this is only a 3rd gear issues. Lack of fuel? Ignition issues? Valve float?