OK, I'm back from my "little medical vacation..."
You need to look at the screen with a magnifying glass and see if it is a tiny checker board pattern that shifts and then freezes or is is it grey lines Raster. Both are different problems.
In either case - the logic board should be recapped.
In the first case (if you see a shifting checkerboard that freezes) That means problem with the ROM more than the RAM. If there was an error with the RAM, it would give you a Sad Mac Error. But in freezing, that means that it is not going to the proper ROM address. Could be corrosion on the pins (if they are on sockets), or cracked/broken solder joints or traces. If the ROMs is in a socket, you can pry the chip up slightly and push it back down to break the corrode crap and test it to see if it turns on properly. If it does, then this is a temporary fix - congrats! Your next step is to carefully remove the ROM and carefully buff out the pins with some metal polish, steel wool or pencil eraser and blow out the socket with some compressed air. And put the ROM back into place.
If it is the second case, where it is just a grey raster, then there is a lot more to do. First check the analog board and recap it, as this is where most of the time this situation happens. Then check to see if your voltages are spot on, make adjustments as needed. Once the analog board is fixed/recapped, then its the logic board's turn. Washing and recapping the board usually fixes most problems. If not you will need to track down broken traces.
You need to look at the screen with a magnifying glass and see if it is a tiny checker board pattern that shifts and then freezes or is is it grey lines Raster. Both are different problems.
In either case - the logic board should be recapped.
In the first case (if you see a shifting checkerboard that freezes) That means problem with the ROM more than the RAM. If there was an error with the RAM, it would give you a Sad Mac Error. But in freezing, that means that it is not going to the proper ROM address. Could be corrosion on the pins (if they are on sockets), or cracked/broken solder joints or traces. If the ROMs is in a socket, you can pry the chip up slightly and push it back down to break the corrode crap and test it to see if it turns on properly. If it does, then this is a temporary fix - congrats! Your next step is to carefully remove the ROM and carefully buff out the pins with some metal polish, steel wool or pencil eraser and blow out the socket with some compressed air. And put the ROM back into place.
If it is the second case, where it is just a grey raster, then there is a lot more to do. First check the analog board and recap it, as this is where most of the time this situation happens. Then check to see if your voltages are spot on, make adjustments as needed. Once the analog board is fixed/recapped, then its the logic board's turn. Washing and recapping the board usually fixes most problems. If not you will need to track down broken traces.