I second the motion. When you say "dips" in SWR, are you speaking about it being too high? I suspect there is an internal problem, and if you're the original owner, it's warranted for as long as you own it.
By the way, grounding the base of the antenna has no effect on its performance. If it does (did), then something else is amiss.
Your statement about not needing a matching coil at the base leads me to conclude that the ground losses are rather high. Properly mounted, and with the vehicle properly bonded, the input impedance on 75 and 40 meters will be about about 20 and 25 ohms respectfully. That's a 2:1 SWR.
My own HiQ 5/80, which is body mounted (photos are on my web page), measures 18 and 23, and without a matching coil, I couldn't use it.