Arm pump is one of the biggest frustrations for riders, so please recognize that you background does not make this a special issue for you! When it comes to arm pump, it is my opinion that body position on the bike is the real issue. Think about it this way, once your body gets behind the center of the bike or quad (i.e. pegs) your arms are the ONLY thing that can keep you on board.
As you have learned or been taught, you need to "grip" with your legs so that your core, lower back and leg muscle groups can keep you in the attack position over the center of the bike. Even if you are accelerating and leaning back for better hook up, you should be gripping with your legs. Your legs, core and lower back muscles are much larger and can handle a great deal of load before they begin to fatigue - especially compared to your forearms. I know this concept is easier said than done, but by focusing on your major muscle groups responsible for maintaining proper position is the key. If possible, have someone film you so that you can evaluate where you are positioned - you may be surprised where you think you are verses where you actually are!
Another suggestion is to look ahead slightly further than you are comfortable - this is a suggestion that came directly from Jeff Emig when we were hosting an open forum at World Minis a few years back. When you look further ahead, your body simply "absorbs" what is directly underneath you which allow you to maintain your proper position. If you don't see what is coming up, you begin to bounce off of the object directly in front of and then each subsequent one there after - slowing you down and making your positioning on the quad/bike less than ideal.
If you have any film that you would like for me to review, please forward to me at firstname.lastname@example.org