1997 Buick Lesabre, 150,000, 3800 Series II, V-6

Since October I have had dual problems, and repairs, may be related. Replaced Manifold and Gasket because of coolant leak. Had gas tank and sender replaced. 1st Problem, gas gauge does not work properly, garage where it was done replaced sender, still not working. The gauge remains on full for about 300 miles then within 20 miles lowers to 3/4 tank and stays there. The gauge was working properly before the new gas tank. 2nd problem I have a very high idle on startup, if I drive like that, car runs high. The problem usually can be stopped by shutting engine off and restarting, sometimes it takes a couple of tries. Then runs fine. Occasionally, the rpm needle with be very eratic and then return to normal. I have spend quite a bit already, if repairs possibly caused current problems, I will ask that they be fixed or for compensation. The high idle poblem did not start until after manifold and gas tank repair. Thanks so much, I don't want to blindly move adhead.