little truck, big alt, still no power

heya, first time poster here. most of the information looks good, so here is my problem.

i have a 94 toyota pickup. cut through the cab and bed to lay in 4 12" Comp kicker VR's on a 1200.1 JBL amp. also have a pioneer 600 watt on some alpine type r components in the front doors. after installing all this, upgraded the alternator to a mean green alt. (somewhere around 140 amps i think). which is up from the 40 amp stock. oh..by the way, ive also added power windows to the mix.

then upgraded the battery to a yellow top, with 0 gauge ground wires to 3 places (inside bumper, chassis, and body). the alt has a 1 gauge wire to the battery. 0 gauge wiring back to the amps in the bed of the truck. problem is..i STILL have voltage problems. my amps only get 10 volts at best when running..and the amp on the highs sometimes kicks out cause the remote turn on isnt gettin enough voltage. the 0 gauge from the battery to the amps is probably close to 17-19 feet long..could this be causing a problem? as far as i can tell, my next solution is caps (lookin at a 2.4 farad cap) and adding another battery in the back.

any suggestion on how to tackle this problem?

and i can only run my stereo really hard for about 10 mins with the car on before the voltage starts causing strange problems (high kick out for a few seconds, subs sometimes dont hit, they just pop, and severe dimming of lights). if i run the truck for awhile with everything off, helps charge things back up to normal and stuff works for awhile..wow. sorry for the huge post. anything would really help, thanks

If you turn your system off dose the voltage go back up? Also do you know if the alternator has an internal or external voltage regulator? And when the voltage drops dose it come back up when you raise the engines RPM's

hmm..i think i remember checkin the amps with the system off, and it was near 13 volts. i believe the alternator is a internal regulator, because it is supposed to be a stock replacement. the voltage does go up with the RPMs..but only at the battery in the front. at idle, my battery tested at 12.4 volts. revved up the engine and it jumped to 14.2.  at the amps in the back it doesnt seem to make much difference. (about a half volt or so higher than idle). im curious if the cable length has anything to do with it..or if maybe my amp isnt putting out what it is supposed to.  

to answer the 2 quick questions i can - the wiring to the amp is so long because it goes from the battery up front, down under the cab, inbetween the cab and bed through a drain hole, and then through the bed to the amps in the far back. i was thinkin the second battery might help shorten this up some and help that problem. and to answer your other question, when i back off the gas, the voltage does NOT stay at 14.2. it drops to around 12.8. only when the engine is revved up does the voltage go up. gonna check on the regulator on the alt today

thanks for the help so far