Well, the first thing I would do is get a multimeter if you don't have one. I think you can get them for less than $20 at Radioshack, that's where I got mine. Set it to diode test mode. What you want to do is put on end on legs of the big black chip that controls the DVD reading stuff. Basically, you need to check for good connections by doing this. The places you solder to on the back of the board are simply connecting to the big chip on the other side's legs. Touch one leg at a time while you have the other end of the multimeter on the modchip itself. Start with point I and then touch the legs of the chip on the side closest to the bios chip until your meter either beeps or shows resistance. If you hear a beep on one of the legs, and ONLY one of the legs and you're 100% sure you have it connected to the right point on the back of the board, then you're good go with I. Repeat for B. Then, repeat this for every other point on the mod - the bios wires are very important obviously. By repeat, I don't mean touch the big chip - but for the bios for example, touch the leg of the bios chip (not just the end of your wire) and touch the appropriate point on the mod chip. They should all test out fine. Same for reset, SX, A, etc.
If that checks out, make sure you used a thick and short ground wire. Same rule should try to apply to the 3.3v power source.
Next, I think a common mistake people make is that they don't insulate the modchip when putting the case back together. I always use electrical tape on the board between the mod and the ps2 mainboard. I then either tape over the chip or on the bottom of the metal heat shield before screwing it back down to the ps2. I'm very careful about preventing shorts this way.
About wires crossing... I'm not as careful with this as others say I should be. I've been told that wires should only cross once, if at all. Especially the DVD controlling wires. Insulated or not, it's my understanding that they can interfere with each other and possibly cause mechon crashes. Try to keep the wires short and flat with the board, without crossing much.
Also, are you sure when you are screwing the case back together that you're not hitting wires with screws? Always keep wires free and clear of any screw holes on the board.
Have you considered the possibility that the dvd/cd laser is worn? Try cleaning it, and if needed - adjusting it a bit (the white gear(s)), but be very careful when doing this and mark your original position.
If somebody can help me out with alternate solder points for the B & I wires for a V5, that would great. That's what we really need here to help this case I think. Sure makes testing easier with a multimeter
Modding in Denver, CO and surrounding burbs