1991 n/a timing problems

[jerrods91supra]

ok i am new to this forum and need some help. 1991 n/a. this car was given to me by a friend.. freshly rebuilt head, hks stopper head gasket, and i am pretty sure felpro everything else as far as gaskets. i was told that when he set the timing, it was set right. and when he drove it around the neighborhood, maybe 10 minutes, the timing would be advanced to 20 degrees. what will cause the timing to advance itself? right now i have the engine tore down to the head. well, thats the way i got it anyway. there is well over $5000 invested in parts and labor on this car and the body is perfect and i cant see junking it. the car has pretty much new everything as far as engine parts, alt. starter, egr valve, water pump, power steering pump, etc. all from toyota. not cheap auto part store crap. any help would be greatly appreciated. thanks in advance