It sounds like a typical example of sludge in your system. Sludge will more so gather in downstairs rads than in upstairs ones. If you have bled all the radiators, feel the bottoms of the rads. If they are cold at the bottom and hot at the top, then you will need to powerflush. Sludge can also cause excessive boiler noise.
Good practice before powerflushing is to test the system for the level of sludge within the system. Ask the company who will be carrying out the works if they have a Central Heating Water Test Kit, such as from Fernox. This will be another good indication of whether a powerflush is required and the level of expertise of that company.
If you do carry out a powerflush, ensure that a good quality inhibitor, such as Fernox, is added to the system upon completion for further protection of the system.
WARNING: If you have gunbarrel (GB) pipework within your heating system, DO NOT powerflush with an agressive cleanser. It may be better to use a magnet unit, such as the new "MagnaCleanse RapidFlush Filter". This will not use cleansers, but circulate the debris into a magnet to remove. It also has a special drill operated Radiator Agitator to loosen sludge from rads. If you have just copper, then full powerflush.
I have copied and pasted some useful info from my website regarding powerflushing:
PowerFlushing is the process by which the central heating system of a house is flushed clean of rust, sludge and any other debris that may have accumulated.
A high flow/low pressure pumping unit is attached to the heating system circulation pump and a series of strong cleansing chemicals are flushed through the entire system.
Unfortunately PowerFlushing is quite an expensive process and you should only have one if you really need it. So how do you know when your heating system needs to be flushed? There are a few common signs that indicate PowerFlushing is needed and these include:
· Cold spots on the radiators or radiators that don't heat up at all
· Excessively discoloured water when you bleed the radiators
· Excessive noise from the boiler unit or the circulation pump
· Poor or non-existent hot water even when the system has been running for a while
These problems on their own may be able to be rectified without PowerFlushing, however, if you notice more than one sign then it is probably worth considering a PowerFlush.
The Benefits of PowerFlushing
The result of PowerFlushing is that the boiler and the rest of the central heating system become more efficient. This occurs because there is better circulation through the pipe work and the radiators and in turn the boiler doesn't have to work as hard to warm the house and water. Therefore the overall result is that you save money on your heating bills.
The Cost of PowerFlushing
As mentioned above PowerFlushing can be quite an expensive process. The price you pay will depend on a number of factors, such as the size of the heating system and the quantity of cleansing chemicals needed to flush the system clean. On average you will be looking to pay between €300 and €600 for a PowerFlushing and so you need to make sure your system will benefit from the process.
Consequences of Not PowerFlushing
If your heating system does need to be PowerFlushed and you decide not to do it then the problems will almost certainly get worse. The circulation through the pipes and the radiators will slow until very little hot water gets through. This means that the house won't heat up and the boiler will be working for no real reason. The result of this is that energy will be wasted and in the long run money will be wasted as well.
Conclusion
So is PowerFlushing worth it? The answer to this depends on how affected your heating system is. If the entire system is showing signs of bad circulation or you notice more than two of the signs mentioned above then a PowerFlush will probably be worth the money you pay. If, however, only a few radiators are suffering from cold spots then it will probably be much quicker and easier to take the radiators off the wall and flush them separately.