The batteries from your Firebird may not be up to the task of powering the new system. Cells are not all created equal, and there are a few factors that determine how much current a cell can produce without losing a lot of voltage:
1. Cell size. In general, bigger cells can produce more current.
2. Cell quality. It's hard to tell by price, but a "cheap" cell, that is one that is of poor quality, will not produce as much current as a good quality cell. Because your Firebird XL was a low-cost, prepackaged unit, I would expect that it came with the cheapest cells they could source at the time, which means that they are probably of mediochre quality.
3. Cell diameter. Shorter, fatter cells can generally produce more current than longer, thinner cells with the same mAh rating.
Actually, I suspect that factor #2 is the cause of your trouble. Get yourself a quality battery pack, and you'll be fine. Battery packs made with HECELLs (
www.edogfight.com) or KAN1050s (various sources) would be great for this application.
I forgot to mention, the new power system setup undoubtedly draws more current than the old Firebird XL.