subs acting possessed

hi im new to 12volt and i have come accross a problem... i recently bought a 99 yukon denali it already came with a system.. it has a memphis 1000 watt mono amp pushing 2 12' L5s. the amp is under the rear seat, next to the amp is a capacitor. well everything was working fine, then yesterday i removed the memphis amp and i put in a 1001bd punch mono amp.. with the truck off it plays good and everything when i turn on the truck the subs hit a couple off times with the stereo off... as im cruising if i go over a bump or a if im going down a bumpy road the subs hit with the rhythm of the road even if i turn off the stereo... please help this is driving me crazy...

another thing if i move the amp around as im driving the problem does not occur... 

check the wires, power, ground, and expecially those RCA cables, are they cheap ones or good ones?

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99 Blazer LT.   Yellow Top. Big 3. Infinity Kappa Speakers All Around. Jensen CD/DVD flip out. 2 Infinity Kappa Perfect 12DVQs powered by a Alpine PDX600.1 (in one custom box, building a FG box)

... maybe its the new amp?!?!

I dont know how old it is but i do know that its not NEW... because thats an older amp model

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99 Blazer LT.   Yellow Top. Big 3. Infinity Kappa Speakers All Around. Jensen CD/DVD flip out. 2 Infinity Kappa Perfect 12DVQs powered by a Alpine PDX600.1 (in one custom box, building a FG box)

well they are streetwire but it was not long enough so theyput an extension and those were cheap but i already replaced that with some tsunami.. also one more thing the capacitor has speaker wire running from it from the terminals like if it used to be connected to something... so its like an open ground connection and a power but  they are taped up

thats what i had thought at first but i had the amp connected to my other truck and i had no problem what so ever and that was recently i took it out of that truck and connected it to the yukon