Improve ventilation or relocate unit to cooler location.
Reduce load; reduce amperes by improving power factor with capacitors; check for circulating currents for paralleled transformers - different ratios or impedances; check for open phase in delta bank.
Change circuit voltage, taps.
If other than naturally cooled, check fans, pumps, valves and other units in cooling systems.
Winding failure - incipient fault
See "No voltage - unsteady voltage" below.
Test for exciting current and no-load loss; if high, inspect core, remove and repair; check core bolt, clamps and tighten; check insulation between laminations; if welded together, return to factory for repair or replacement.
High harmonic loads
Measure neutral current - replace with K-rated transformer