Inconsistent response from Bass Shaker

Hello,

Please help. I have just installed 4 Dayton BST-1 bass shakers, a SUCA M-504C 4 x 50w amp and a super cheap (no name) external sound card. Each of the 4 channels on the amp has its own gain and each is set to about 50%. The amp is not hot to the touch. I'm running a licensed version of Simhub and playing ACC. I originally had 5 vibration effects going to the bass shakers, gear shift, abs, traction control, engine noise and road rumble. Something didn't seem quite right with the vibration effects, so I turned them all off in simhub except the gear shift. What I noticed is that sometimes...the timing of the gear shift vibration effect is spot on, sometimes it's delayed, sometimes it doesn't happen at all and sometimes rather than the effect being a short 100 millisecond burst it's a sustained burst for 3-4 seconds. Can anyone help me out here and tell me what I've done wrong to cause this? Any help would be much appreciated!