Slumping automotive sales are sending jitters through the industry and have economists scratching their heads. Inflation is low, at 5.8% unemployment is relatively stable and consumers are confident about the economy Still, April sales were dismal - an anemic 13.9-million rate. That sobering number forced the Big Three to downscale their annual sales forecasts and cut production of overloaded models. Big Three economists say the sales slowdown is a fluke. In a mid-month forecast, Ward's Automotive Reports indicated May light-vehicle sales would rebound by around 12% from the April setback - but still would lag a tad behind May '94. Some experts even blame the IRS for the April slide, citing the agency's slow processing of tax return checks. Whatever, automakers are cranking up rebates and low-interest deals with a roar to recapture sales momentum. Chrysler Corp. is tempting buyers of sluggish-selling LHS models with $2,500. More surprising, Chrysler also is offering cash back on its recently introduced JA models, Cirrus and Stratus. The good news for manufacturers is they're still selling trucks as fast as they bolt them together.