electrical problem please help

[fourNSpyder]

this all started with my egt guage, it began to bounce around uncontrollably one day. It will read perfectly normal with the car not running and the key in, but soon as i start it it goes crazy. I disconnected it thinking that maybe the probe was going bad. Now when I have my headlights on and the heater on my tachometer will jump around. This only seems to happen when the fan kicks on, or the headlights are on with the heater on. I went to autozone and had the charging systewm tested. They said my alternator was bad. I replaced it with a remanufactured one from autozone. Same problem. I read on some other boards that those alternators have been known to not have the right voltage regulators in them so i went and got one from the dealership. Again same problem. I have also tried replacing the battery but they didnt help either. The only other thing I can think of to try would be a homemade ground kit which i will be doing here shortly. Does anyone else have any thoughts on this. Has this ever happened to anyone else? My car is a 99 eclipse spyder with a 4g64 in it, also i have tried swapping the ecu with a 2g gst ecu since i have put a turbo kit on. this made no difference either. The car has begun to idle surge as well, but will stop the idle surge after it has been driving for a while. I am stumped please help, thanks
Tony

 

[Spln_Hrd]

its only happening on aftermarket stuff...

where is the power source for the malfuntioning stuff?

what is your stock tach doing when your aftermarket one is going crazy? also when your gauges are malfuntioning. is your tach reading below and too the rpm it should read or above and too the rpm or above and below? also the same question for the egt gauge, is it reading higher, lower, or lower and higher than the actual tempurature?

 

[fourNSpyder]

The malfuncting tach is the stock one in the guage cluster, sorry i forgot to mention that. I do not have an aftermarket tach. it only seems to go up from the correct rpm, just kinda bounces up from the real rpm. As i said before it only seems to do this when i have some accessories turned on, such as the headlights, and heater/defroster. The stereo will also do it if i turn it up, i do have a sub and amp in the trunk. I have tried pullin th fuse for that so that it wouldnt turn on, but the other things still make it happen. I would think it is a ground issue so i am going to try that homemade ground kit that is listed under the tech articles. any other ideas?