1996 Toyota Tacoma Engine Stalls When Shifting

When I shift into any gear, at any speed, I occasionally feel a deep lagging in the engine. I get a major loss in power. It feels as though I'm trying to shift into 2nd gear from a standstill. I will slow down, not just coast. Sometimes the engine will stall. I seem to still have electrical power (lights on, etc.). The check engine light has now flashed on. All fluids seem fine, although I do not know how to check the transmission fluid.