Photoshop and WP5540 compatibility

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I have a two-year-old Manhattan brand WP5540 pen tablet. It’s not fancy at all, it was like $25. I had problems with it and tried reinstalling the drivers when I got Photoshop CS4. Well, the tablet is now working, but it is still having serious issues within Photoshop. I searched around thousands of help sites and followed numerous helpful hints including:
-Plugging the tablet into the USB on the back of my computer (This temporarily solved my problem of straight lines when I was drawing a curved line, but the problem came back.)
-Setting specific settings and restarting Photoshop (This works, but only for one stroke; after I draw the first line, the settings are once again screwed up.)
-Right-clicking the Photoshop icon, going to properties, then Compatibility and checking the box ‘Turn off advanced text services for this program’ (I have no idea what it did. I thought it helped, but it certainly didn’t solve my problem.)

The actual current problem is this:
After starting Photoshop, opening a file and setting my brush settings to ‘Pen Pressure’ (under Shape Dynamics-Size Jitter-Control), I can make one line exactly the way it should be, with me controlling the line thickness with my pen. After that first stroke, however, any attempts at drawing with the same settings produces marks so tiny and light that you cannot see them without zooming in about 700%. If I change any of the settings and change it back to pen pressure, it still makes unbelievably miniscule marks. (Photoshop is recognizing the attempt, though, because the layer I’m working on in the Layers palette flickers every time I make a mark. I originally thought that no marks were being made at all, until I zoomed in and tested it.) If I switch the Control to off, I have the problem that no matter how long, loopy and curvy I attempt to make my line, it simply makes a straight line from the point where I started my line to the point where I ended it. If I restart, the whole process starts over with the one single pressure sensitive line and the phooey to follow.
Something I just found out: The above was all done using a size 3 brush. After everything is screwy, if I change the brush size to, say, 15px, and press very hard with my pen, a (once again straight) stroke so small you can hardly see it is made. Pressing very hard with the pen and swiping it quickly across the tablet produces straight lines of varying widths. I guess it is still pressure sensitive, but what is with the eternally straight lines?
If I change the settings to Pen Pressure (under Shape Dynamics-Size Jitter-Control), click the little lock icon beside the Shape Dynamics option, close Photoshop and reopen it, open a new file and select a brush and draw a line, it works exactly how it should, for a time. The weirdest thing about my problem is that the results are random every time. I tried restarting Photoshop with the settings locked and tried the brush, and it was pressure sensitive for two strokes before shifting to the ‘straight, tiny line syndrome’. I tried this again and was able to make maybe fifty marks before everything went straight. I once again tried switching to a larger brush (32px) and all the lines were once again perfectly straight, and seemed to be responding in some way to the pressure sensitivity, but it was utterly random. Like, I would press really hard in a straight line and it would taper from a small tip to a large end. Then I tried swirling the pen around while still pressing hard on the pen I would get maybe a 1px wide straight line with no varying widths. I tried doing the same exact motions on the tablet with my pen and would get differing lines every time. Does my Photoshop have a bug? Could it perhaps be a bug not yet worked out of the new CS4 program? Or is it my tablet not being compatible?

The mouse always works when I try using brushes with it, but it doesn’t of course give me the tablet brush effect of sensitivity.

I have the driver that came on the installation disc with my tablet installed currently. I don’t know if that’s out of date, and if it is I don’t know how to get a current one.

What the heck is the matter? My last resort will be investing in a Wacom.