Hello
The power supply regulation section in the circuit board of your TV is faulty. There is a switching power supply regulation section, called [SMPS], generally, which regulates the main input voltage to lower supply voltages of different volts, and feeds the different sections circuit boards of the TV. This section is composed of one seperate circuit board, which will have bulky capacitors and switching transfomers. You can detect this board very easly by following the main power supply line wire which goes diretly to it after the internal ON/OFF switch of the TV. Normally, this board is a non-repairable one, which has only limited components that can be replaced, by normal soldering techniques.
If any loose solderings in this supply board will show ths problem as you describes here. Open your TV, locate this board. Check for any loose soldering points are there in its soldering side, and is there any bulged capacitors in it. If there are any bulge capacitors, you can replace it with same type and in capacitance; as it is a repairable job with this board. It is best to resolder most of the suspected soldering points by applying a little more solder, without making any solder bridge short in between adjecent solder points. If this does not help you replace the power supply regulator board with same type and number. This baord will have a number, printed on it, and must be noted when buying a new one. Can be bought it from most of the electronic spare part shops at your town. Picture of a typical power sypply board of an LCD TV is shone here for reference. The board at your TV may not be the same type, but will contain all the components shown here in this board. So you can locate very easly the power board of your TV. OK