1990 Toyota Camry Loss of Power

My Camry doesn't have the power it used to have. I've recently replaced the plugs and cap and wires, fuel filter, evap canister as it was all plugged up. It idles perfectly. I can live with it not have the power it once had however it sometimes has hardly any power at all. It is wound out and barely goes 20mph. I can get back it it the next day and its back like it was. I noticed I changed the air filter once and then it had no power which lead me to believe there may be a wiring problem with mass air intake. But don't think that's it either. I replaced the O2 sensor, didn't help. Had the castalytic convereter checked with a temperature sensor gun and was told it was ok. Not sure if that's the best test. My next step is to drop the fuel tank and check the filter and pump unless some of you guys have any better ideas. It's an automatic and trans axle fluid level is correct. Fuel injectors seem to be working normal when I listen to them with stethoscope. I'm just a DIY and by know means an expert though. I've read every thread on every site and no one seems to have the same problem. Oh, it seems to have a whistling like sound coming from around the exhaust manifold at head. I slight puff of smoke when I rev the engine by hand while looking under the hood. Could that be the head gasket, but if that was it why would I have normal (albeit not that great power) and then have no power for a short time again. I love this old car as it gets twice the mpg as my late model F150. There are some very smart and helpful guys answering questions on this site and enjoy it very much. Thanks for all that you do!