If no ice dams or mold inside on the roof sheathing in 40 years, that's a pretty good track record. I would air seal the attic from all holes in the top plates, etc. before adding more insulation: http://www.rd.com/how-to-seal-attic-...icle18158.html
Seal the attic hatch access door to be certain the attic fan doesn't draw the house conditioned air out along with the gas unit's back-draft. http://www.nachi.org/attic-ventilati...s-part1-25.htm
If in a hot climate, the attic venting will help with your AC's loads: http://www.buildingscience.com/docum...on?full_view=1
Most roofing manufacturers require venting for their warranty.
Some roofs shouldn't be vented, depends on conditions and location: http://www.fpl.fs.fed.us/documnts/pdf1999/tenwo99a.pdf
Clothes taking longer to dry?
Clean the dryer screen in HOT water if using fabric softener sheets.
They leave a residue that impedes air-flow, costing you money.
Clean the ducting in the last six months? 17,000 dryer fires annually!