air keeps getting in coolant

[1stgsr]

so a little history first.....the eng i have in my db8 is the one that was in my previous dc2. and in both cars, the coolant temp on the gauge always stayed low (maybe one or two notches above the cool mark) well just recently the temp is rising more than normal and its bc i keep getting air in the lines. the upper radiator hose is full of pressure so i open the bleeder, go through the flush/bleed air procedure ect. my cooling fan is working, i cannot find any leaks, when i get off wrk im going to check the t-stat, and go frm there.

any ideas or advice on where or what to check for????